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Appendix D: Glossary

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Introduction

Some terms and definitions within this appendix have been obtained from the Texas Board of Professional Land Surveying (TBPLS) and/or the Texas Society of Professional Surveyors.

This appendix contains the definitions of most, but not all GPS and survey related terms. The terms and their descriptions below are explained as used/intended in the context of this manual. Other or more elaborate descriptions may exist for the terms listed.

Other Definitions – Terminology used in this manual shall be defined herein, or when not defined herein, shall refer to the Definitions of Surveying and Associated Terms: (1978), as compiled by the joint committee of the American Society of Civil Engineers and the American Congress on Surveying and Mapping.

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1

1-sigma – 1-sigma is one standard error from the mean.

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A

accidental error – An error for which it is equally probable that the sign of the error is a plus or minus value; an error for which there is no proportional change or relationship between measurements, conditions and the sign or magnitude of the error; an error, evident in a series of measurements, which is compensated in total effect.

accuracy – Accuracy is how close measurements are to the accepted value of truth.

accuracy of the bearing (or course) – In relation to source, accuracy of the bearing (or course) is the relationship of each bearing as expressed on a map, plat and/or in a description of the new survey shall not exceed the angular relationship of the stated source by more than the following tolerance:

  • Sin = (approximately) and rounded to nearest 5 seconds
  • Where = + bearing accuracy in seconds (rounded)
  • p = the denominator of the allowable error of closure (precision) for the particular Condition (i.e. 5,000; 7,500; 10,000 or 15,000)

adjusted values – Adjusted values are the values derived from observed data (measurement) by applying a process of eliminating errors in that data in a network adjustment.

adjustment – Adjustment is the process of determining and applying corrections to observations for the purpose of reducing errors in a network adjustment.

adjustment convergence – Adjustment convergence occurs when the network adjustment has met the defined residual tolerance or last ditch residual tolerance within a defined number of iterations.

algebraic sign – An algebraic sign is a sign (+ or -) associated with a value which designates it as a positive or negative number.

algorithm – An algorithm is a set of rules for solving a problem in a finite number of steps.

almanac – An almanac is data transmitted by a GPS satellite which includes orbit information on all the satellites, clock corrections, and the atmospheric delay parameters. This data is used to facilitate rapid SV acquisition within GPS receivers.

ambiguity – Ambiguity is the unknown integer number of cycles of the reconstructed carrier phase contained in an unbroken set of measurements. The receiver counts the radio waves (from the satellite as they pass the antenna) to a high degree of accuracy. However, it has no information on the number of waves to the satellite at the time it started counting. This unknown number of wavelengths between the satellite and the antenna is the ambiguity. Ambiguity is also known as integer ambiguity or integer bias.

angular closure – Angular closure for each condition is expressed as the number of seconds allowable for any angle multiplied by the square root of the number of angles in the traverse. This value should not be exceeded in any loop closure. The basis for this angular value is well documented in standard textbooks on surveying practice and procedures.

antenna height – Antenna height is the height of a GPS antenna phase center above the point being observed. The uncorrected antenna height is measured from the observed point to a designated point on the antenna, and then corrected to the true vertical manually or automatically in the software.

antenna phase center – Antenna phase center is the electronic center of the antenna. It often does not correspond to the physical center of the antenna. The radio signal is measured at the Antenna Phase Center.

antenna phase correction – Antenna phase correction is the antenna phase correction is the phase center for a GPS antenna is neither a physical nor a stable point. The phase center for a GPS antenna changes with respect to the changing direction of the signal from a satellite. Most of the phase center variation depends on satellite elevation. Modeling this variation in antenna phase center location allows a variety of antenna types to be used in a single survey. Antenna phase, center corrections are not as critical when two of the same antennas are used since common errors cancel out.

anti-spoofing (AS) – Anti-spoofing is a feature that allows the U.S. Department of Defense to transmit an encrypted Y-code in place of P-code. Y-code is intended to be useful only to authorized (primarily military) users. AS is used with selective availability to deny the full precision of GPS to civilian users.

autonomous positioning – Autonomous positioning is a mode of operation in which a GPS receiver computes position fixes in real time from satellite data alone, without reference to data supplied by a base station. Autonomous positioning is the least precise positioning procedure a GPS receiver can perform, yielding position fixes that are precise to ±100 meters horizontal RMS when selective availability is in effect, and to ±10-20 meters when it is not. This is also known as absolute positioning and point positioning.

azimuth – The azimuth is a surveying observation used to measure the angle formed by a horizontal baseline and geodetic north. When applied to GPS observations, it refers to a normal section azimuth.

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B

base station – A base station is an antenna and receiver set up on a known location. It is used for real-time kinematic (RTK) or differential surveys. Data can be recorded at the base station for later Postprocessing. In GPS surveying practice, the user may observe and compute baselines (that is, the position of one receiver relative to another). The base station acts as the position from which all other unknown positions are derived.

baseline – A baseline is the position of a point relative to another point. In GPS surveying, this is the position of one receiver relative to another. When the data from these two receivers is combined, the result is a baseline comprising a three-dimensional vector between the two stations.

bearing source – The source of the bearing (or course) must be stated in the report, on the Plat or in any description as one of the following:

  • geodetic bearing
  • grid bearing of the Texas Coordinate System of 1983 (or 1927), with the proper zone specified, Sec. 21.071, et seq., Tex. Nat. Res. Code Ann. (Vernon 1978), or
  • record bearing or the relation thereto, along a line monumented on the ground as called for in said record.

bench marks – A bench mark is a relatively permanent object, natural or man-made, bearing a marked point, whose elevation above or below an adopted datum is known. Usually designated as “BM,” such a mark is sometimes further qualified as a PBM (permanent bench mark) or as a TBM (temporary bench mark). Often, a TxDOT disk is set in a concrete monument or drilled into a concrete surface or object is used.

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C

C/A code – The standard (Course/Acquisition) GPS code. A sequence of 1023 pseudorandom, binary, biphase modulations on the GPS carrier at a chip rate of 1.023 MHz. It is also known as the “civilian code.” C/A code helps the receiver compute the distance from the satellite.

CAF – Combined Adjustment Factor. CAF is the product of the scale factor and the elevation factor. The CAF times the surface distance yields the corresponding distance on the state plane grid.

calculation of area - accurate and carried to – The perimeter courses and distances as shown on a map, plat or drawing representing the survey shall compute to the area stated on the Plat. The decimal shall be carried only so far as it is compatible with the precision of the survey and not beyond the last significant number. A one-acre survey with a precision of 1:5,000 will result in an area calculation of + value of 0.0002 acre. The acreage should then be carried only to the nearest 0.001 acre. Likewise a survey of a one-acre tract with a precision of 1:15,000 will result in an area calculation of + 3 square ft. or about + .00007 acre. The acreage can then be carried to the nearest .0001 acre. Similar values can be mathematically applied to any size tract by the formula:

  • Ae = A - (e/(e+1) x A)
  • Where: Ae = + probable error in square feet
  • A = area of tract in square feet, determined from survey
  • e = denominator of error of closure for the particular Conditions (5,000; 7,500; 10,000; 15,000).

carrier – A carrier is a signal that can be varied from a known reference by modulation.

carrier frequency – Carrier frequency is the frequency of the unmodulated fundamental output of a radio transmitter.

Cartesian coordinates – See fixed earth-centered-earth Cartesian coordinates.

category – Category is a unit dividing major professional services of a RPLS into defined segments of similar nature, procedure and practice. A Category is comprised of one or several services or products that are closely allied. A Route Survey is a Category. A Land Title Survey is a different Category. Each Category has specific requirements.

CBN – Cooperative Base Network. CBN consists of B order stations set in cooperation with various governmental agencies for the purpose of densifying the National Spatial Reference System.

channel – A channel of a GPS receiver consists of the circuitry necessary to tune into the signal from a single GPS satellite.

chi-square test – Chi-square is an overall statistical test of the network adjustment. It is a test of the sum of the weight squares of the residuals, the number of degrees of freedom and a critical probability of 95 percent or greater. The purpose of this test is to reject or to accept the hypothesis that the predicted errors have been accurately estimated.

clock bias – Clock bias is the difference between the clock’s indicated time and true universal time.

clock offset – Clock offset is the constant difference in the time reading between two clocks. In GPS, usually refers to offset between SV clocks and the clock in the user’s receiver.

closure – Closure is a mathematical application whereby a determination is made as to the exactness that a geometrical form is generated or attained within its confined elements of connecting lines and points. It is a computation method used by a land surveyor to test the quality of field survey measurements and to apply corrections in balancing or adjusting the survey to meet precision specifications.

coarse acquisition – A course acquisition is a pseudorandom noise (PRN) code modulated onto an L1 signal.

code – Code is the GPS code and is a pseudorandom noise (PRN) code that is modulated onto the GPS carrier signals. The C/A code is unclassified and is available for use by civilian applications. The P code is also known and unclassified, but may be encrypted for national defense purposes. Code measurements are the basis of GPS navigation and positioning. Code also is used in conjunction with carrier phase measurements to obtain more accurate survey quality baseline solutions.

collection rate – The rate at which a receiver collects SV data.

component – A component is one of the three surveying observations used to define a three-dimensional baseline between two control points. The same baseline can be defined by azimuth, delta height, and distance (in ellipsoid coordinates); by delta X, delta Y, and delta Z in (Earth Centered Cartesian coordinates); and by delta north, delta east, and delta up (in local plane coordinates).

constant systematic error – A Constant systematic error is represented by conditions that do not change during a series of measurements.

constellation – A constellation is a specific set of satellites used in calculating positions: three satellites for 2-D fixes, four satellites for 3D fixes. It is all of the satellites visible to a GPS receiver at one time. The optimum constellation is the constellation with the lowest PDOP.

constrained – Constrained is a way to hold (fix) a quantity (observation and coordinate) as true in a network adjustment.

constraint – Constraint is external limitations imposed upon the adjustable quantities (observations and coordinates) in a network adjustment.

contour interval – A contour interval is a predetermined difference in elevation (vertical distance) at which contour lines are drawn. The contour interval is usually the same for maps of the same scale.

contour line – A contour line is an imaginary line on the ground, all points of which are above or below a specified datum.

contour map – A contour may is a map that portrays relief by means of contour lines.

control – A control is a system of points whose relative positions have been determined from survey data.

controlling monument – A monumented land corner to which a land survey is referenced.

control point – A point that has a very accurate coordinate. This may be a published NGS point or one that was surveyed by other means. This is the type of point that a reference receiver should be placed on.

control stations – Control stations are stations whose position (horizontal or vertical) has been determined from survey data and is used as a base for a dependent survey.

control survey – A control survey is a survey that provides positions (horizontal or vertical) of points to which supplementary surveys are adjusted.

conventional observation – A conventional observation is an observation in the field obtained using a total station or theodolite.

coordinates – Coordinates are linear or angular quantities, or both, which designate the position on a point in relation to a given reference frame.

correlated – When observations are correlated, there are two or more observations (or derived quantities), which have at least one common source of error.

CORS - Continuously Operating Reference System – CORS is a network of the highest quality horizontal stations, forming the National Spatial Reference System and providing the public with 24-hour raw GPS data.

covariance – Covariance is a measure of the correlation of errors between two observations or derived quantities. Covariance also refers to an off-diagonal term (that is, not a variance) in a variance-covariance matrix.

covariance matrix – A matrix that defines the variance and covariance of an observation. The elements of the diagonal are the variance and all elements on either side of the diagonal are the covariance.

covariant values – This is the publication of the propagated (computed) a posteriori errors in azimuth, distance, and height between pairs of control points resulting from a network adjustment. The term covariant indicates that this computation involves the use of covariant terms in the variance-covariance matrix of adjusted control points.

cycle slips – A discontinuity in the measured carrier beat phase resulting from a temporary loss of lock in the carrier loop of a GPS receiver.

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D

data collector – A data collector is a handheld electronic field notebook. It connects to a total station, level, or GPS receiver to receive and temporarily store raw data.

data logging – Data logging is the process of recording satellite data in a file stored in the receiver, a data collector, or on a PC card.

data message – A message included in the GPS signal which reports the satellite’s location, clock corrections, and health.

datum – Datum is a mathematical model of the earth designed to fit part or all of the geoid. It is defined by the relationship between an ellipsoid and a point on the topographic surface established as the origin of the datum. It is usually referred to as a geodetic datum. The size and shape of an ellipsoid, and the location of the center of the ellipsoid with respect to the center of the earth, usually define world geodetic datums.

datum grid/multiple regression – Datum grid/multiple regression are datum transformations, usually convert data collected, in the WGS-84 datum (by GPS methods) onto datums used for surveying and mapping purposes in individual regions and countries.

datum transformation – Datum transformation defines the transformation that is used to transform the coordinates of a point defined in one datum to coordinates in a different datum. There are a number of different datum transformation methods including seven-parameter and three-parameter (or Molodensky).

de-correlate – To de-correlate is to remove the covariances between observations. This may be done through elaborate orthogonal transformations, or by computing separate horizontal and vertical adjustments.

deflection of the vertical – A deflection of the vertical is the angular difference between the upward direction of the plumb vertical line (vertical) and the perpendicular (normal) to the ellipsoid.

degrees of freedom – Degrees of freedom is a measure of the redundancy in a network.

delta elevation – Data elevation is the difference in elevation between two points.

delta N, delta E – Delta N and delta E are coordinate differences, expressed in a Local Geodetic Horizon delta U coordinate system.

delta X, delta Y, delta Z – Delta X, Y, and Z are coordinate differences, expressed in a Cartesian coordinate system.

differential positioning – Precise measurement of the relative positions of two receivers tracking the same GPS signals.

discrepancy – The difference between two measurements of the same quantity.

DOP – Dilution of Precision is a measure of the accuracy of a GPS position based on the relative positions of the satellites. DOP is an indicator of the quality of a GPS position. It takes account of each satellite's location relative to the other satellites in the constellation, and their geometry in relation to the GPS receiver. A low DOP value indicates a higher probability of accuracy.

Standard DOPs for GPS applications are:

  • PDOP Position (three coordinates)
  • HDOP Horizontal (two horizontal coordinates)
  • VDOP Vertical (height only)
  • TDOP Time (clock offset only).

doppler shift – A doppler shift is the apparent change in frequency of a signal caused by the relative motion of satellites and the receiver.

double differencing – Double differencing is an arithmetic method of differencing carrier phases simultaneously measured by two receivers tracking the same satellites. This method removes the satellite and receiver clock errors.

DTM – Digital Terrain Model. DTM is a representation in graphic form, on the computer, of the terrain through the area being surveyed.

dual-frequency – A dual frequency is a type of receiver that uses both L1 and L2 signals from GPS satellites. A dual-frequency receiver can compute more precise position fixes over longer distances and under more adverse conditions because it compensates for ionospheric delays.

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E

earth-centered-earth – An earth-centered-earth is a Cartesian coordinate system used by the WGS-84 reference frame.

easting – Easting is an eastward reading of grid values. Easting is read left to right on a grid (X-axis).

elevation – is the vertical distance of a point above or below a datum plane.

elevation mask – This is the angle above the horizon, below which satellite signals are not used.

ellipsoid – The Earth is neither perfectly smooth or round. Earth scientists (geodesists) have mathematically smoothed the surface of the Earth by averaging the highs and lows. This new calculation is called an ellipsoid. GPS uses WGS 84 as its ellipsoid base.

The ellipsoid is a mathematical model of the earth formed by rotating an ellipse around its minor axis. For ellipsoids that model is the earth, the minor axis is the polar axis, and the major axis is the equatorial axis.

  • An ellipsoid is defined by specifying the lengths of both axes, or by specifying the length of the major axis and the flattening.
  • Two quantities define an ellipsoid; these are usually given as the length of the semi-major axis, a, and the flattening, where b is the length of the semi-minor axis.

ellipsoid distance – An ellipsoid distance is the length of the normal section between two points. Ellipsoid distance is not the same as the geodesic distance.

ellipsoid height – An ellipsoid height is the distance, measured along the normal, from the surface of the ellipsoid to a point.

ephemeris – An ephemeris is a set of data that describes the position of a celestial object as a function of time. Each GPS satellite periodically transmits a broadcast ephemeris describing its predicted positions through the near future, uploaded by the control segment. Postprocessing programs can also use an ultra rapid, rapid or precise ephemeris, which describes the exact positions of a satellite in the past.

epoch – An epoch is the time interval when the receiver logs data to its memory. An epoch is the measurement interval of a GPS receiver. The epoch varies according to the survey type.

epoch date – The epoch date is the date, usually expressed in decimal years for which published coordinates and data are valid.

epoch interval – Epoch interval is the measurement interval used by a GPS receiver; also called a cycle.

error – An error is the difference between the measured value of a quantity and its true value. Surveying errors are generally divided into three categories: blunders, systematic errors, and random errors. Least squares analysis is used to detect and eliminate blunders and systematic errors, and least squares adjustment is used to measure and properly distribute random error.

error ellipse – An error ellipse is a coordinate error ellipse is a graphical representation of the magnitude and direction of the error of network adjusted points.

error of closure – Reflects the precision of the survey and is the result of mathematically determining the latitude and departures and subsequently the misclosure of the traverse. Once this value has been determined and found to be of no lesser quality than required, any suitable adjustment may be made.

events – Events are represented as a record of the occurrence of an event, such as the closing of a photogrammetric camera’s shutter. A GPS receiver can log an event mark containing the time of the event and an alphanumeric comment entered through the keypad to describe the event. An event can be triggered through the keypad or by an electrical signal input on one of the receivers’ ports.

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F

fast ambiguity resolution – Fast ambiguity resolution is rapid static or fast static GPS surveying techniques, utilizing multiple observables (dual-frequency carrier phase, C/A and P codes) to resolve integer ambiguities, with shortened observation periods. The method may also be used for observations with the receiver in motion known as on-the-fly ambiguity resolution.

faststatic – FastStatic is a method of GPS surveying using occupations of up to 20 minutes to collect GPS raw data, then postprocessing to achieve sub-centimeter precisions. Typically the occupation times vary based on the number of satellites (SVs) in view. FastStatic is also referred to as RapidStatic.

FBN – Federal Base Network – An FBN consists of A and B order stations set by the NGS for the purpose of densifying the National Spatial Reference System.

FCA – An FCA is a fully constrained adjustment.

feature codes – Feature codes are abbreviations used to define an object collected during a radial survey.

FGCS - Federal Geodetic Control Subcommittee

FGDC – Federal Geodetic Data Committee.

fixed – See constrained.

fixed (ECEF) – Earth-centered-earth fixed (ECEF) is a Cartesian coordinate system used by the WGS-84 reference frame. In this coordinate system, “fixed” refers to the center of the system that is at the earth’s center of mass. The z-axis is coincident with the mean rotational axis of the earth and the x-axis passes through 0× N and 0× E. The y-axis is perpendicular to the plane of the x and z-axes.

fixed coordinates – Fixed coordinates are point coordinates that do not move when performing a network adjustment.

fixed elevation – A fixed elevation is an elevation obtained, either as a result of tide observations or previous adjustment of leveling, which is held at its accepted value in any subsequent adjustment.

fixed position – A fixed position is an adjusted value of the position of a point on the earth. The positions obtained by the adjustment are called adjusted positions, and when used a control for other survey work they are called fixed positions.

fixed solution – A fixed solution is a solution obtained when the baseline processor is able to resolve the integer ambiguity search with enough confidence to select one set of integers over another. It is called a fixed solution because the ambiguities are all fixed from their estimated float values to their proper integer values.

flattening – A flattening is a mathematical expression of the relative lengths of the major and minor axes of an ellipsoid.

flattening inverse – A flattening inverse is an expression of the flattening that is easier to read and edit.

float solution – A float solution is a solution obtained when the baseline processor is unable to resolve the integer ambiguity search with enough confidence to select one set of integers over another. It is called a float solution because the ambiguity includes a fractional part and is non-integer.

free adjustment – Performs a network adjustment in which no point (coordinate) is constrained. The network adjustment uses inner constraints.

frequency – Frequency is the size and spread of residuals in a data set; graphically shown in distribution histograms.

fully constrained – Fully constrained is a network adjustment in which all points in the network that are part of a larger control network are held fixed to their published coordinate values. Fully constrained is used to merge smaller with larger control networks and old to newer networks.

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G

GDOP - Geometric Dilution of Precision – GDOP is the relationship between errors in user position and time and errors in satellite range. See also DOP.

geodetic azimuth – A geodetic azimuth is the angle between the geodetic meridian and the tangent to the geodesic line of the observer, measured in the plane perpendicular to the ellipsoid normal of the observer; clockwise from north.

geodetic datum – A geodetic datum is a mathematical model designed to fit part or all of the geoid. It is defined by the relationship between an ellipsoid and a point on the topographic surface established as the origin of a datum.

  • The size and shape of an ellipsoid and the location of the center of the ellipsoid with respect to the center of the earth define world geodetic datums. Various datums have been established to suit particular regions.
  • For example, European maps are often based on the European datum of 1950 (ED-50). Maps of the United States are often based on the North American Datum of 1927 or 1983 (NAD-27, NAD-83). All GPS coordinates are based on the WGS-84 datum surface.

geographic (geodetic) coordinates – Latitude, longitude, and ellipsoid height.

geoid – A geoid is an Earth model that takes into account the Earth’s gravity field. Geodesists have recalculated the Earth’s surface so that an object does weigh the same no matter where it is placed.

A geoid is the surface of gravitational equipotential that closely approximates mean sea level. It is not a uniform mathematical shape, but is an irregular figure.

  • Generally, the elevations of points are measured with reference to the geoid. However, points fixed by GPS methods have heights established in the WGS84 datum (a mathematical figure).
  • The relationship between the WGS-84 datum and the geoid must be determined by observation, as there is no single mathematical definition that can describe the relationship. The user must utilize conventional survey methods to observe the elevation above the geoid, and then compare the results with the height above the WGS84 ellipsoid at the same point.
  • By gathering a large number of observations of the separation between the geoid and the WGS84 datum (geoidal separation), grid files of the separation values can be established.
  • This allows the interpolation of the geoidal separation at intermediate positions. Files containing these grids of geoidal separations are referred to as geoid models. Given a WGS84 position that falls within the extents of a geoid model, the model can return the interpolated geoidal separation at this position.

geoid model – A geoid model is a mathematical representation of the geoid for a specific area, or for the whole earth. The software uses the geoid model to generate geoid separations for the user’s points in the network.

geoid separation – Geoid separation is the distance between the ellipsoid and geoid at a given point.

geomatics – Geomatics is the design, collection, storage, analysis, display, and retrieval of spatial information. The collection of spatial information can be from a variety of sources, including GPS and terrestrial methods. Geomatics integrates traditional surveying with new technology-driven approaches, making geomatics useful for a vast number of applications.

GPS - Global Positioning System – A GPS is based on a constellation of twenty-four (24) satellites orbiting the earth at a very high altitude.

GPS baseline – A GPS baseline is a three-dimensional measurement between a pair of stations for which simultaneous GPS data has been collected and processed with differencing techniques. This baseline is represented as delta X, delta Y, and delta Z; or azimuth, distance, and delta height.

GPS observation – A GPS observation is an uninterrupted collection of GPS data at a particular point in the field. A number of observations are done simultaneously in a session to create baselines by processing the data.

GPS raw data – GPS raw data is the data collected by a GPS receiver for the purpose of processing at a later time.

GPS time – GPS time is a measure of time used by the NAVSTAR GPS system. GPS time is based on Universal Time Coordinated (UTC) but does not add periodic leap seconds to correct for changes in the earth’s period of rotation.

GPS week – A GPS week is an incremental number of weeks, starting at 0 hour UTC on the date January 6, 1980. April 6, 1007 is the first day of GPS week 900.

gravity void – A gravity void is a block or area of blocks within the gravity measurement database without observations. A geoid model relying upon this database would be weak and possibly in error at these blocks.

grid – A grid is a two-dimensional horizontal rectangular coordinate system, such as a map projection.

grid azimuth – A grid azimuth is measured from grid north.

grid conversion – A grid conversion is the conversion between geographic and map projection coordinates.

grid coordinates – Grid coordinates are the numbers of a coordinate system that designates a point on a grid.

grid declination - The angular difference in direction between grid north and true north at any given place.

grid position – Grid position are the grid coordinates of a point.

ground control – Ground control, in photomapping, is the control obtained from surveys as distinguished from control obtained by photogrammetric methods.

grid distance – The grid distance is the distance between two points that is expressed in mapping projection coordinates.

ground distance – Ground distance is the distance (horizontal distance with curvature applied) between two ground points.

ground plane – A ground plane is a large flat metal surface, or electrically charged field, surrounding a GPS antenna used to deflect errant signals (multipath) reflected from the ground and other near-by objects.

GRS 80 guard stake – A GRS 80 guard stake is a stake driven near a point usually sloped with the top of the guard stake over the point. The guard stake protects, and its markings identify the point.

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H

HDOP – Horizontal Dilution of Precision.

height measurement – A height measurement is a measuring tool supplied with an external GPS antenna and used rod for measuring the height of the antenna above a point.

H. I. – Measurement from point on the ground to the antenna of either the base or rover receiver.

histogram – A histogram is a graphical display of the size and distribution of residuals in a network adjustment.

horizontal – A horizontal is a point with horizontal coordinate accuracy only. The control point elevation or ellipsoid height is of a lower order of accuracy or is unknown.

horizontal control survey – is performed for the purpose of placing geographic coordinates of latitude and longitude on permanent monuments for referencing lower levels of surveys. A projection is used to place the coordinates on a plane of northing and easting values for simplified measurements. Scale and elevation factors are applied to make the distance measurements applicable to the exact project location on the working surface and the type of projection chosen is an “equal angle” type. The required datum for all TxDOT surveys is NAD 83 (HARN adjustment) unless otherwise directed by TxDOT.

horizontal datum – A datum is a set of precisely surveyed points on the ground that have been referenced to a given ellipsoid.

horizontal distance – A horizontal distance is the distance between two points, computed horizontally from the elevation of either point.

horizontal position – A horizontal position is a point with horizontal coordinates only.

HTDP – Horizontal Time Dependent Positioning model. HTDP is a computer database and interpolation program developed by NGS to predict horizontal displacements between coordinate points over time. The program can work backwards in time where it includes earthquake parameter or forward in time where only the secular motion is analyzed.

hub – A hub is a wooden stake set in the ground, with a tack or other marker to indicate the exact position. A guard stake protects and identifies the hub.

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I

independent – An independent is the subnetworks, observations, and control points not connected by geometry or errors. This term is the opposite of correlated.

independent baseline – An independent baseline is a non-trivial baseline. Those vectors determined from differencing common phase measurements only once. For any given session there are n – 1 independent vectors where n is the number of receivers operating.

inner constraint – An inner constraint is a network adjustment computed without fixing any point coordinates.

instrumental errors – From imperfections or faulty adjustment of the instruments or devices with which measurements are taken.

integer ambiguity – Inner ambiguity is the whole number of cycles in a carrier phase pseudorange between the GPS satellite and the GPS receiver.

integer search – Integer search is the GPS baseline processing, whether real-time or postprocessed, requires fixed integer solutions for the best possible results. The software which processes the GPS measurements used to derive a baseline does an integer search to obtain a fixed integer solution. The search involves trying various combinations of integer values and selecting the best results.

iono free – Ionospheric free solution (IonoFree). IonoFree is a solution that uses a combination of GPS measurements to model and remove the effects of the ionosphere on the GPS signals. This solution is often used for high-order control surveying, particularly when observing long baselines.

ionosphere – The band of charged particles 80 to 120 miles above the Earth’s surface.

ionosperic delay – An ionospheric delay is a signal delay or acceleration as a wave propagates through the ionosphere. Phase delay depends upon the electron content and affects the carrier signal. Group delay depends upon the dispersion in the ionosphere as well, and affects the code signal.

ionospheric modeling – Ionospheric modeling is the time delay caused by the ionosphere varies with respect to the frequency of the GPS signals and affects both the L1 and L2 signals differently. When dual frequency receivers are used the carrier phase observations for both frequencies can be used to model and eliminate most of the ionospheric effects. When dual frequency measurements are not available an ionospheric model broadcast by the GPS satellites can be used to reduce ionospheric affects. The use of the broadcast model, however, is not as effective as the use of dual frequency measurements.

iteration – An iteration is a complete set of adjustment computations that includes the formation of the observation equations, normal equations, coordinate adjustments, and computation of residuals.

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K

K reduced column – K reduced column is an abbreviated version of the normal equations in which the profile equations are reordered to minimize the computer memory required to store all nonzero elements.

kinematic surveying – Kinematic surveying is a method of GPS surveying using short Stop and Go occupations, while maintaining lock on at least 4 satellites. It can be done in real-time or postprocessed to centimeter precisions.

known point initialization – A known point initialization is used in conjunction with kinematic initialization. If two known points are available, the baseline processor can calculate an inverse between the two points and derive an initialization vector. This initialization vector, with known baseline components, is used to help solve for the integer ambiguity. If the processor is able to successfully resolve this ambiguity a fixed integer solution is possible, yielding the best solutions for kinematic surveys.

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L

L1 – L1 is the primary L-band carrier used by GPS satellites to transmit satellite data. Its frequency is 1575.42 MHz. It is modulated by C/A code, P code, and a Navigation Message.

L2 – L2 is the secondary L-band carrier used by GPS satellites to transmit satellite data. Its frequency is 1227.6 MHz. It is modulated by P code and a Navigation Message.

land survey, boundary survey or property survey – A survey performed by a RPLS for the primary purpose of locating, describing, monumenting and mapping a parcel of land.

land title survey – A survey of real property performed by a RPLS to be used by a title insuring agency for purposes of insuring title to said real property.

latitude – 1.) latitude the angular distance north or south of the equator. 2.) latitude, in plane surveying, is the amount that one end of a line is north or south of the other end. As the plane coordinates of a point are known as the easting and northing of the point, the latitude is the difference between the northings of the two ends of the line, which may be either plus or minus.

least squares – A mathematical method for the adjustment of observations, based on the theory of probability. In this adjustment method, the sum of the squares of all the weighted residuals is minimized.

level of confidence – A level of confidence is a measure of the confidence in our results, expressed in a percentage or sigma.

level datum – A level datum is a level surface to which elevations are referred. The generally adopted level datum for leveling in the United States is mean sea level. For local surveys, an arbitrary level datum is often adopted and defined in terms of an assumed elevation for some physical mark (bench mark).

leveling – is the operation of measuring vertical distances, directly or indirectly, to determine elevations.

level net – Level net are lines of spirit leveling connected together to form a system of loops or circuits extending over an area. Level net is also called a vertical control net.

level of significance – A level of significance is an expression of probability. A one-sigma (standard) error is said to have a level of significance of 68 percent. For one-dimensional errors, a 95 percent level of significance is expressed by a 1.96 sigma, and a percent level of significance is expressed by a 2.576 sigma.

local ellipsoid – A local ellipsoid is the ellipsoid specified by a coordinate system. The WGS-84 coordinates are first transformed onto this ellipsoid; then converted to grid coordinates.

local geodetic – A local geodetic is the latitude, longitude, and height of a point. The coordinates are those expressed in terms of the local ellipsoid. A local geodetic is at any point, a plane at the ellipsoid height of a given point, which is horizontally parallel to the tangent plane to the ellipsoid at that point. Coordinate values for the local geodetic horizon are expressed as North, East, and Up. The LGH is used for rotating EC Cartesian Coordinate differences, before modeling a baseline on the ellipsoid. Azimuth values computed from LGH components must be corrected for skew normals as part of modeling on the ellipsoid.

loop closure – Loop closures provide an indication as to the amount of error in a set of observations within a network.

  • A loop closure is calculated by selecting a point from which one or more observations were taken, adding one of those observations to the point’s coordinates, and calculating coordinates of the second point based on that observation.
  • This process is repeated one or more times around a loop, finally ending at the original starting point. If there were no errors in the observations, the final calculated coordinate would be exactly the same as the original starting coordinate.
  • By subtracting the calculated coordinate from the original coordinate a misclosure is determined. Dividing this error by the length of the line allows the error to be expressed in parts per million.
  • This technique can also be used between two different points when both points are known with a high degree of accuracy. This is also known as a traverse closure.
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M

mapping angle – Mapping angle is the angle between grid north on a mapping projection and the meridian of longitude at a given point. Also know as convergence.

major axis – See ellipsoid.

major collector – A major collector is an area public way for purposes of vehicular travel, usually interstate or federal highways; includes the entire area within the right of way.

map projections – These are representations of the Earth’s features that are transferred to a flat two-dimensional plane, such as, paper maps and computer generated maps.

mapping projection – Mapping projection is a rigorous mathematical expression of the curved surface of the ellipsoid on a rectangular coordinate grid.

mask angle – Cut-off angle. A mask angle/cut-off angle is the point above the observer’s horizon below which satellite signals are no longer tracked and/or processed. Ten to twenty degrees is typical.

MCA – minimally constrained adjustment.

mean sea level – A mean sea level is the mean height of the surface of the ocean for all stages of the tide. Used as a reference for elevations.

meridian – A meridian is a north-south line from which longitudes (or departures) and azimuths are reckoned.

minimally constrained – A minimally constrained network is a network adjustment in which only enough constraints to define the coordinate system are employed. It is used to measure internal consistency in observations.

minor axis – See ellipsoid.

mistake or blunder – An unintentional fault of conduct arising from poor judgment or from confusion in the mind of the observer.

modeling – Modeling is the expressing of an observation and its related errors mathematically and geometrically on some defined coordinate system, such as an ellipsoid.

monument – A monument is any object or collection of objects (physical, natural, artificial) that indicates the position on the ground of a survey station.

multipath – A multipath is an interference (similar to ghosts on a television screen) that occurs when GPS signals arrive at an antenna after traveling different paths. The signal traveling the longer path yields a larger pseudorange estimate and increases the error. Multiple paths may arise from reflections from structures near the antenna.

multipath errors – Errors caused by the interference of a signal that has reached the receiver antenna by two or more different paths. This occurs when the antenna is placed too close to a large object, such as water towers, overhead storage tanks, etc.

multi-channeled receiver – A GPS receiver that can simultaneously track more than one satellite signal.

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N

NAD27 – North American Datum of 1927 referenced to the Clark 1866 ellipsoid.

NAD83 – North American Datum of 1983 referenced to the GRS80 ellipsoid.

NAD83/HARN93 – This datum is more accurate than NAD83 because it is based on a more accurate network of control points. To avoid confusion it should be considered a separate datum from NAD83.

narrow-lane – A narrow-lane is a linear combination of L1 and L2 carrier phase observations (L1 + L2) that is useful for canceling out ionospheric effects in collected baseline data. The effective wavelength of the narrow-lane is 10.7 centimeter.

natural or external errors – From variations in the phenomena of nature such as temperature, humidity, wind, gravity, refraction, and magnetic declination.

NAVD88 – North American Vertical Datum of 1988.This vertical datum is the most recent and most accurate.

NAVDATA – NAVDATA is the 1500-bit navigation message broadcast by each satellite. This message contains system time, clock correction parameters, ionospheric delay model parameters, and details of the satellite’s ephemeris and health. The information is used to process GPS signals to obtain user position and velocity.

negligent error – The difference between the measurement and the true value in a given quantity.

network – A network is a set of baselines. See also subnetwork.

network adjustment – A network adjustment is a solution of simultaneous equations designed to achieve closure in a survey network by minimizing the sum of the weighted squares of the residuals of the observations.

network status – Network status is an indication that a particular observation will be included in the adjustment. Network means that it is included in the adjustment and non-network means that it is excluded from the adjustment.

NGS - National Geodetic Survey

NGVD29 – This vertical datum is mostly outdated, but is still used in the USGS 7.5 minute quadsheets.

NMEA – National Marine Electronics Association. The NMEA 0183 Standard defines the interface for marine electronic navigational devices. This standard defines a number of strings referred to as NMEA strings that contain navigational details such as positions.

NSRS – National Spatial Reference System.

normal – A normal in geodesy is the straight line perpendicular to the surface of the ellipsoid.

normal distribution curve – A normal distribution curve is a graphical illustration of the theoretical distribution of random variables around an expected value according to probability theory. It is used with histograms.

northing – A northing is a northward reading of a grid value.

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O

observation – An observation is an uninterrupted collection of GPS data at a particular point in the field. A number of observations are done simultaneously in a session to create baselines by processing the data.

observation residual – An observation residual is the correction applied to an observation, as determined by the adjustment.

occupation time – An occupation time is the amount of time required on a station, or point, to achieve successful processing of a GPS baseline. The amount of time will vary depending on the surveying technique, the type of GPS receiver used, and the precision required for the final results. Occupation times can vary from a couple of seconds (kinematic surveys) to several hours (control or deformation surveys that require the highest levels of precision and repeatability).

occupied station – An occupied station is a traverse or triangulation station over which a theodolite or an engineer transit is set up for the measurement of angles at this station. It is also a station at which angles have been measured.

offset line – An offset line is a supplementary line close to, and usually parallel to a main survey line to which it is referenced by measured offsets. When the line for which data is desired is in such position that it is difficult to measure over it, the required data is obtained by running an offset line in a convenient location and measuring offset from it to salient points on the other line.

OPUS – On-line Positioning User Service (OPUS) provides GPS users easier access to the National Spatial Reference System (NSRS) and is operated by the National Geodetic Survey (NGS). OPUS allow users to submit their GPS data files to NGS, where the data will be processed to determine a position using NGS computers and software. Each data file that is submitted will be process with respect to 3 CORS sites and the results are e-mailed to the user.

OPUS-RS – OPUS-RS is a version of OPUS designed to obtain geodetic quality positioning results from user data sets as short as 15 minutes. The reference station selection algorithm for OPUS-RS differs from that for regular OPUS. OPUS-RS searches the reference stations in order of increasing distance from the user’s station (the rover), selecting reference stations that have suitable data.

order of accuracy – An order of accuracy is a mathematical ratio defining the general accuracy of the measurements made in a survey. The orders of accuracy for surveys are divided into four classes named: first-order, second-order, third-order, and fourth-order.

origin – an origin is the intersection of axes in a coordinate system. It is the point of beginning.

orthometric height – An orthometric height is the distance between a point and the surface of the geoid. It is usually called the elevation.

OTF search method – On-the-fly (OTF) search method is a GPS baseline processing, whether real-time or postprocessed, requires fixed integer solutions for the best possible results.(See integer search.)

  • Historically, this search was done using measurements collected while two or more receivers were stationary on their respective points. Modern receivers and software can use the measurements collected while the roving receiver is moving. Because the receiver is moving, the data is described as collected On-the-fly (OTF) and the integer search using this data is an OTF search.

outlier – An outlier is an observation which is identified by statistical analysis as having a residual too large for its estimated error. The term derives from the graphical position of an observation in a histogram.

over-determined – An over-determined network is a network for which more measurements have been made than are necessary to compute the coordinates of the network. It is related to redundancy.

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P

parallax – A parallax is a change in positions of the image of an object with respect to the telescope cross hairs when the observer’s eye is moved. This can be practically eliminated by careful focusing.

parameter – A parameter is an independent variable in terms of which the coordinates of points on a line or surface are given.

parity – Parity is a form of error checking used in binary digital data storage and transfer. Options for parity checking include even, odd, or none.

P-code – The Precise or Protected code. A very long sequence of pseudo-random binary biphase modulations on the GPS carrier at a chip of 10.23 MHz which repeats about every 267 days. Each one-week segment of the code is unique to one GPS satellite and is reset each week.

PDOP – Position Dilution of Precision is an indication of the current satellite geometry. A PDOP is a unitless figure of merit expressing the relationship between the error in user position, and the error in satellite position. It is the result of a calculation, which takes into account each satellite’s location relative to the other satellites in the constellation. A low PDOP indicates a higher probability of accuracy. Usually a PDOP of 6 or below gives excellent positions.

Geometrically, PDOP is proportional to 1 divided by the volume of the pyramid formed by lines running from the receiver to four satellites that are observed. Values considered “good” for positioning are small, for example 3. Values greater than 7 are considered poor. Thus, small PDOP is associated with widely separated satellites.

PDOP is related to horizontal and vertical DOP by:

PDOP2 = HDOP2 + VDOP2

PDOP cutoff – A receiver parameter specifying a maximum PDOP value for positioning. When the geometric orientation of the satellites yields a PDOP greater than the mask value, the receiver stops computing position fixes.

PDOP mask – A PDOP mask is the highest PDOP value at which a receiver will compute positions.

Personal Errors – This error occurs from the limitation of the human senses of sight and touch.

phase center models – Phase center models are models used to apply a correction to a GPS signal based on a specific antenna type. The correction is based on the elevation of the satellite above the horizon and models electrical variations in the antenna phase center location. These models are useful for eliminating errors introduced when identical antennas are not used at both the base and rover points. See also antenna phase correction.

phase difference processing – Relative positioning. Phase difference processing is a computation of the relative difference in position between two points by the process of differencing simultaneous reconstructed carrier phase measurements at both sites. The technique allows cancellation of all errors which are common to both observers, such as clock errors, orbit errors, and propagation delays. This cancellation effect provides for determination of the relative position with much greater precision than that to which a single position (pseudorange solution) can be determined.

PI – A PI is the Point of Intersection of back tangent and forward tangent.

picture point – A picture point in surveying is a terrain feature that is easily defined on an aerial photograph. Its horizontal or vertical positions have been determined by survey measurements. Picture points are marked on the aerial photographs by the surveyor and are used by the photomapper.

plane coordinates – See grid coordinates.

plane survey – A plane survey is a survey in which the effect of the curvature of the earth is almost entirely neglected, and computations of the relative positions of the stations are made using the principles of plane geometry and plane trigonometry.

plumbing – Plumbing is the act of aligning the antenna or instrument along a vertical line (plumb line) perpendicular to the equipotential surface of earth’s gravity field.

point positions – See autonomous positioning.

positions – Positions are the place occupied by a point on the surface of the earth. Positions are data that defines the location of a point with respect to a reference system.

positional tolerance – A measure of the accuracy of the position of a monumented boundary corner with respect to its described location without error.

positional tolerance of any monument – The distance that any monument may be mislocated due to normal imperfect measurements opposed to its actual location by nearly perfect measurement.

  • This value can be determined by dividing the length of any course of a closed traverse by the denominator of the required error of closure. The results of this calculation will establish the tolerance or radius around a point.
  • No traverse adjustment shall be made to any distance larger than this positional error. If the measurements are checked with a one-second theodolite and a recently calibrated distance-measuring device of known high accuracy, the values must fall within the tolerance or radius calculated.
  • All distances between monuments from 0 through 200 feet shall have a positional error not to exceed 0.03 feet. This short distance may be checked with a recently standardized tape, taking temperature, tension and sag into consideration.
  • It is important to allow for the elapsed time between the date of any measurement and the date of any check made on that measurement in order to consider climatic conditions, man-made or other disturbances that may have affected the monuments.

post-processing – A procedure used to obtain accurate coordinates by correcting errors in the rover receiver data. This is accomplished by processing the rover receiver data with the base receiver data. The rover receiver and the base receiver must run concurrently and include the same satellites.

PPM - Parts per million – PPM is a standardized representation of a scale error in distance measurements. A 1 PPM error would result in 1 millimeter of measurement error for every 1000 meters of distance traveled.

precise ephemeris – See ephemeris.

precision – How close multiple measurements of a single point are to each other.

prime meridian – A prime meridian is the initial or zero median from which longitudes are reckoned. At an international conference in 1884, the Greenwich Meridian was adopted by most countries as the prime meridian for the earth.

prime vertical – A prime vertical is a vertical circle perpendicular to the plane of the celestial meridian. The plane of the prime vertical cuts the horizon in the east and west points.

PRN – Pseudorandom number – 1.) A sequence of digital 1’s and 0’s that appear to be randomly distributed like noise, but that can be exactly reproduced. PRN codes have a low autocorrelation value for all delays or lags except when they are exactly coincident. 2.) Each NAVSTAR satellite can be identified by its unique C/A and P pseudorandom noise codes, so the term PRN is sometimes used as another name for GPS satellite or SV.

probability – Probability is a statistical percentage expressing what portion of a hypothetical number of observations will fall within the defined limits. It is sometimes called level of significance.

probable value – A probable value is the adjusted value for observations and other quantities, assuming that the adjustment has been done correctly. It is the closest approximation to true value that is possible.

professional surveying – from Section 2 of the TBPLS Practices Act means the practice for compensation of land, boundary, or property surveying or other similar professional practices.

  • The term includes any service or work the adequate performance of which involves the application of special knowledge of the principles of mathematics, related applied and physical sciences, and relevant laws to the measurement and location of lines, angles, elevations, natural features, and existing man-made works, and, on the beds of bodies of water, the determination of areas and volumes, for:
    • the location of real property boundaries
    • the platting and layout of lands and subdivisions of land
    • the preparation and perpetuation of maps, record plats, field note records, and real property descriptions that represent those surveys.

projection – A projection is used to create flat maps that represent the surface of the earth or parts of the Earth’s surface.

propagated error – A propagated errors are computed errors derived from estimated observational errors and expressed in terms of coordinate positions. Propagated coordinate errors may, in turn, be propagated into relative errors in azimuth, distance, and delta height between points.

pseudorange – A pseudorange is a measure of the apparent propagation time from the satellite to the receiver antenna, expressed as a distance. The apparent propagation time is determined from the time shift required to align a replica of the GPS code generated in the receiver with the received PGS code.

  • The time shift is the difference between the time of signal reception (measured in the receiver time frame) and the time of emission (measured in the satellite time frame). Pseudorange is obtained by multiplying the apparent signal-propagation time by the speed of light.
  • Pseudorange differs from the actual range by the amount that the satellite and receiver clocks are offset, by propagation delays, and other errors including those introduced by selective availability.

pseudostatic GPS – Pseudostatic GPS, also known a pseudo-kinematic and repeat occupation, is a relative positioning technique which relies upon two or more simultaneous observations at a point pair, separated by some time interval (typically 60 minutes or more), in order to solve the integer bias terms from the change in satellite geometry occurring between the repeat observations.

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Q

quality acceptance test – A quality acceptance test is one or more software evaluation tests, performed on raw GPS measurement data, to determine if the data passes or fails a set of tolerance values that the user defines. These tests either remove data from further processing or mark data requiring quality improvements.

QC records – Quality Control records. QC records are used with precise positioning applications. This receiver option allows a user to process RTCM-104 corrections and satellite data in real time to provide position precision statistics.

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R

ratio – A ratio is used during initialization. The receiver determines the integer number of wavelengths for each satellite. For a particular set of integers, it works out the probability that it is the correct set.

  • Ratio is the ratio of the probability of correctness of the currently best set of integers to the probability of correctness of the next-best set.
  • Thus, a high ratio indicates that the best set of integers is much better than any other set. This gives us confidence that it is correct. The ratio must be above 5 for new point and OTF initializations.

RDOP – Relative Dilution of Precision.

real-time corrections – Real-time DGPS uses a data link (beacon or commercial) to transmit correctional data from the reference to the rover receiver. These corrections are used by the rover receiver to correct its errors as the satellite signal is received. No post-processing is required to obtain positions corrected to meter level accuracy.

real-time kinematic – Real-time kinematic is a method of GPS surveying in real-time using short (stop and go) occupation, while maintaining lock on at least 4 satellites. The real-time kinematic method requires a wireless data link between the base and rover receivers.

record – Any documentary material filed in the public records of a city, county or state office that pertains to the location of real property.

rectangular – A rectangular are coordinates in any system in which the axes of reference intersect coordinates at right angles.

redundancy – Redundancy is the amount by which a control network is over-determined, or has more observations than are needed to strictly compute its parts.

redundancy number – Redundancy number is a measure of the degrees of freedom in a portion, rather than the entirety, of a control network.

redundant baselines – Redundant baseline is a baseline observed to a point that has already been connected to the network by other observations. A redundant baseline can be either an independent re-observation of a previous measurement, or an observation to a point from another base. It is redundant because it provides more information than is necessary to uniquely determine a point. Redundant observations are very useful. They provide a check on the quality of previous measurements.

redundant – Redundant is a repeated observation or an observation which contributes to over-observation determining a network.

reference factor – See standard error of unit weight.

reference frame – A reference frame is the coordinate system of a datum.

reference station – A reference station is a base station.

reference variance – A reference variance is the square of the reference factor.

Registered Professional Land Surveyor – means an individual registered as a registered professional land surveyor by the TBPLS.

relative errors – A relative errors are errors and precisions expressed for and between pairs of network-adjusted control points.

relative precision – A relative precision is defined as a measure of the tendency of a set of numbers to cluster about a number determined by the set (e.g. the mean). The usual measure is the standard deviation with respect to the mean.

  • Relative precision denotes the tendency for the various components (X, Y, Z) between one station and other stations in the network to be clustered about the adjusted values.
  • Current custom is to express relative precision at the two-standard deviation (95% confidence) level. This may be stated in terms of a relative error ellipse or as a proportion of the separation distance (e.g. 10 ppm or 1:100,000).

residual – A residual is the correction or adjustment of an observation to achieve overall closure in a control network. It is also, any difference between an observed quantity and a computed value for that quantity.

RINEX – Receiver INdependent EXchange format – A RINEX is a standard GPS raw data file format used to exchange files from multiple receiver manufacturers. An interchange format that permits data collected by one specific receiver to be read by another vendor’s receiver.

RMS – Root Mean Square – A RMS expresses the accuracy of point measurement. It is the radius of the error circle within which approximately 68% of position fixes are found. It can be expressed in distance units or in wavelength cycles.

RMSE – Root Mean Square Error.

rotated meridian – A rotated meridian is a zone constant for the oblique Mercator mapping projection.

rotation – In transformations, a rotation is an angle through which a coordinate axis is moved around the coordinate system origin.

rover – A mobile GPS receiver that when used in conjunction with a stationery receiver can obtain differentially corrected ground coordinates. Any receiver used in a dynamic mode is called a rover.

ROW – Right of Way

regional reference point (RRP) – TxDOT presently operates 47 Regional Reference Points that are positioned across the state.

RTCM – Radio Technical Commission for Maritime Services. RTCM is a commission established to define a differential data link for the real-time differential correction of roving GPS receivers.

RTK – A real-time kinematic is a type of GPS survey.

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S

SAF – Surface Adjustment Factor. SAF is a published TxDOT-developed value for each county which, when multiplied times a distance on the state plane grid, yields the corresponding distance on the surface.

satellite constellation – The arrangement in space of a set of satellites.

satellite geometry – A satellite geometry is a position and movement of GPS satellites during a GPS survey.

satellite vehicle (SV) – A Department of Defense satellite orbiting the Earth.

scalar – In least squares, a scalar is a value applied to the variances (errors) based on the required level of confidence.

scale – A scale is a multiplier used on coordinate and other linear variables, such as for map projections and transformations.

SCOS - Standing Committee on Surveying

SDMS – Survey Data Management System. SDMS ® is a data collection and processing software based on a set of format definitions. AASHTO maintains this system.

secular motion – A secular motion is that portion of crustal motion which is continuous and at a constant velocity. Secular motion is uniformly predictable over time and is independent of any seismic events.

selective availability (S/A) – SA is an artificial degradation of the GPS satellite signal by the U.S. Department of Defense. The error in position caused by S/A can be up to 100 meters.

semimajor axis – Semimajor is one-half of the major axis.

semiminor axis – Semiminor is one-half of the minor axis.

session – A session is a period during which a number of GPS receivers log satellite data simultaneously for the purpose of creating baselines.

set-up error – Set-up errors are errors in tribrach centering or height of instrument at a control point.

sideshot – A sideshot is an observed baseline with no redundancy.

sigma – Sigma is a mathematical symbol or term for standard error.

signal to noise ratio (SNR) – An indicator of the strength of a satellite signal.

single-frequency – Single-frequency is a type of receiver that only uses the L1 GPS signal. There is no compensation for ionospheric effects.

site calibration – Site calibration is a process of computing parameters which establishing the relationship between WGS-84 positions (latitude, longitude and ellipsoid height) determined by GPS observations and local known coordinates defined by a map projection and elevations above mean sea level. The parameters are used to generate local grid coordinates from WGS-84 (and vice-versa) real-time in the field when using RTK surveying methods.

skyplot – A skyplot is a polar plot that shows the paths of visible satellites for the time interval selected for the graph. The elevation of the satellite is represented in the radial dimension and the azimuth is shown in the angular dimension. The result depicts the satellite’s path as it appears to an observer looking down from a place directly above the survey point.

slope distance – A slope distance is the distance in the plane parallel to the vertical difference (slope) between the points.

SNR – Signal-to-Noise Ratio.

solution types – Solution types refer to a description of both the data and techniques used to obtain baseline solutions from GPS measurements.

  • Typical solution types include descriptions such as code, float, and fixed. These describe techniques used by the baseline processor to obtain a baseline solution.
  • Solution types also may include descriptions such as L1, L2, wide-lane, narrow-lane, or ionospheric free. These describe the way the GPS measurements are combined to achieve particular results.

standard deviation – A standard deviation is a standard error. Surveying applications use the conventional formula for sample standard deviation. Standard deviation is a measure of the strength of a satellite signal. SNR ranges from 0 (no signal) to around 35.

standard error – A standard error is a statistical estimate of error, according to which 68 percent of an infinite number of observations will theoretically have absolute errors less than or equal to this value.

standard error of unit weight – A standard error of unit weight is a measure of the magnitude of observational residuals in an unit weight adjusted network as compared to estimated pre-adjustment observational errors.

State Plane Coordinate System (Texas) – Consists of five zones North (4201), North Central (4202), Central (4203), South Central (4204), and South (4205). Each zone is referenced to its own axis.

static (surveying) – Static is a method of GPS surveying using long occupations (hours in some cases) to collect GPS raw data, then postprocessing to achieve sub-centimeter precisions.

static network – A static network is a network that describes the geometry and order in which GPS baselines collected using static and fast static techniques are organized and processed.

  • The baseline processor first examines the project for points with the highest quality coordinates, and then builds the processing network from those points. The result is a set of static baselines that are derived using accurate initial coordinates.

status – Status is every observation and set of keyed-in coordinates for a point has a status field (available in the Summary page of the Properties window). The status can be enabled, enabled as check, or disabled:

  • Enabled observations and coordinates are always used by recomputation in determining the calculated position for the point.
  • Enabled as check observations and coordinates are only used if there are no Enabled ones Disabled observations and coordinates are never used.

stochastic model – A stochastic model is a general reference to the techniques used to estimate errors in a network adjustment.

substantial compliance – The compliance or conformity with essential requirements. Also the equivalent of substantial performance, where inconsequential, trivial variations or omissions are minimized, but may occur.

surveyor, land surveyor or registered professional land surveyor – A person holding a valid license to practice land surveying as a Registered Professional Land Surveyor (RPLS) in the state of Texas, as issued by the Texas Board of Professional Land Surveying (TBPLS).

survey observation – A survey observation are the measurements made at or between control points using surveying equipment (conventional or GPS).

SV – satellite space vehicle.

systematic error – An error which, for known changes in measurement conditions, results in proportional changes of values which remain unchanged, both in magnitude and sign. This error, evident in a series of measurements, may be instrumental, personal, or natural and always follows some definite mathematical or physical law and is cumulative in total effect.

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T

target – A target is any object to which the instrument is pointed. A target may be a plumb bob or cord, a nail in the top of a stake, a taping arrow, a range pole, a pencil, or any other object that will provide a sharply defined, stationary point or line. A target usually placed vertically over an unoccupied transit station.

tau (value) – A tau is a value computed from an internal frequency distribution based upon the number of observations, degrees of freedom, and a given probability percentage (95%).

  • This value is used to determine if an observation is not fitting with the others in the adjustment. If an observation’s residual exceeds the tau, it is flagged as an outlier. Tau values are known as tau lines in the histogram of standardized residuals; vertical lines left and right of the center vertical line.

tau criterion – A tau criterion is Allen Pope’s statistical technique for detecting observation outliers.

TBPLS - Texas Board of Professional Land Surveying

TDOP – Time Dilution of Precision.

terrestrial observation – A terrestrial observation is an observation in the field using a laser rangefinder or conventional instrument.

Texas Statewide Mapping System (TSMS) – This is a standardized statewide (uninterrupted by zones) projection. TSMS possesses more map distortion than the zoned projection.

tie – A tie is a survey connection from a point of known position to a point whose position is desired.

tolerance – The allowable imperfection of any value stated or established in a survey. Each category has four conditions, and each condition has certain tolerances or specifications for values that must be met. The following explanations of tolerance items are to be used with the tolerance chart for each category.

total station – A total station is an electronic theodolite that provides both angle and distance measurements and displays them automatically.

total systematic error – In any given number of measurements is the algebraic sum of the individual errors of the individual measurements.

TOW – Time of Week. TOW is measured in seconds from midnight Saturday night/Sunday morning GPS time.

tracking – The process of receiving and recognizing signals from a satellite.

transformation – A transformation is the rotation, shift, and scaling of a network to move it from one coordinate system to another.

transformation group – A transformation group is a selected group of observations used to compute transformation parameters unique to that group of observations. Typically, the observations within the group are the same type with similar errors and measured using a common method.

transformation parameters – Transformation parameters is a set of parameters derived for a network adjustment or user-parameters defined, that transform one datum to another. Typically, with GPS the parameters are generated to transform WGS-84 to the local datum.

transit station – A mark over which the instrument is, has been, or will be accurately positioned for use.

tribrach – A tribrach is a centering device used for mounting GPS antennas and other survey instruments on survey tripods.

tribrach centering errors – Tribrach centering errors are errors associated with centering (plumbing) the tribrach over errors the observed point. These errors are estimated. The estimate is based on surveying the quality of surveying methods and should be conservative.

tropo correction – tropospheric correction. Tropo correction/tropospheric correction is the correction applied to a satellite measurement to correct for tropospheric delay.

tropo model – tropospheric model – A tropo model occurs when GPS signals are delayed by the troposphere.

  • The amount of the delay will vary with the temperature, humidity, pressure, height of the station above sea level, and the elevation of the GPS satellites above the horizon.
  • Corrections to the code and phase measurements can be made using a tropo model to account for these delays.

TSPS - Texas Society of Professional Surveyors

turning points – Turning points are temporary points of known elevation.

TxDOT - Texas Department of Transportation.

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U

UDN – User Densification Network. A UDN is a station set by the public that have been “bluebooked” by the NGS for the purpose of providing additional control stations adjusted to the National Spatial Reference System.

univariate – Univariate is a mathematical function describing the behavior of one-dimensional random errors in angle, distance; difference in height, elevation, or ellipsoid height.

  • angle
  • distance
  • difference in height
  • elevation
  • ellipsoid height

universal time – Universal time is local solar mean time at Greenwich Meridian.

  • UT0 – Universal Time as deduced directly from observations of stars and the fixed numerical relationship between Universal and Sidereal Time; 3 minutes56.555 seconds.
  • UT1 – UT0 corrected for polar motion.
  • UT2 – UT1 corrected for seasonal variation in the earth’s rotation rate.
  • UTC – Universal Time Coordinated; uniform atomic time system kept very closely to UT2 by offsets. Maintained by the U.S. Naval Observatory. GPS time is directly relatable to UTC. UTC-GPS = 9 seconds (in 1994).

Universal Transverse Mercador (UTM) – A projection created by the U.S. Army to obtain a series of maps that would encircle the Earth. Texas is covered by zones 13, 14, and 15.

unknowns – The computed adjustments to coordinates and transformation parameters; also used to compute observation residuals.

URA – User Range Accuracy. URA is the contribution to the range-measurement error from an individual error source (apparent clock and ephemeris prediction accuracies), which is converted to range units; assuming that the error source is uncorrelated with all other error sources.

US National Geodetic Survey – This is the United States government agency that maintains the national geodetic datum and all geodetic survey control networks within the US and its territories.

U.S. Survey Foot – 1200/3937 meter. The official unit of linear measure for NAD-27.

UTC – Universal Time Coordinated. UTC is a time standard based on local solar mean time at the Greenwich meridian.

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V

variance – The square of the standard error.

variance factor – Reference variance, variance of unit weight. A statistical measure of how close the observation residuals match the predicted errors.

  • It is the square root of the sum of the weighted squares of the residuals divided by the degrees of freedom. If the errors in a network have been weighted correctly, the variance factor will approach 1.0.

variance component – A least-squares technique for estimating the relative error estimation of different portions of a network.

variance group – A variance group is one of the groups of observations for which variance component estimation is being used in a network adjustment.

variance-covariance – A variance-covariance is the set of numbers expressing the variances and covariances matrix in a group of observations.

variable systematic error – Conditions change, resulting in corresponding changes in the magnitude of the error.

VDOP – Vertical dilution of precision.

vector – A vector is a three-dimensional line between two points.

vertical – A vertical is similar to the normal, except that it is computed from the tangent plane to the geoid instead of the ellipsoid.

vertical adjustment – A vertical adjustment is a network adjustment of vertical observations and coordinates only.

vertical control – A Vertical control is an established benchmarks.

vertical control point – A vertical control point is a point with vertical coordinate accuracy only. The horizontal position is of a lower order of accuracy or is unknown.

vertical control survey – is performed in order to accurately determine the orthometric height (elevation) of permanent monuments to be used as bench marks for lower quality leveling.

  • spirit leveling is the usual method of carrying elevations across the country from “sea level” tidal gauges. However, GPS can be used indirectly but with less accuracy
  • eight measurements from the ellipsoid (as opposed to the “sea level” geoid) can be determined very accurately with GPS and only with GPS
  • trigonometric leveling with a total station is not acceptable for vertical control work

vertical datum – A vertical datum is a set of precise levels that have been referenced to a geoid to establish mean sea level.

vertical datum plane – is the level surface to which elevations are referred. In the past, mean (average) sea level was the most common datum used for the United States. Today, the more common reference datum is NAVD 88. This datum is required for all TxDOT surveys unless specifically directed otherwise by TxDOT.

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W

WAAS – Wide Area Augmentation System. WASS is a satellite-based system that broadcasts GPS correction information. WAAS capable GPS receivers can track WAAS satellites. WAAS is synonymous with the European Geostationary Navigation Overlay (EGNOS) and Japan’s Multifunctional Transport Satellite Space-based Augmentation System (MSAS).

water course – A stream of water such as a river, brook, creek, bayou, etc. A watercourse is a visible channel for water such as a ditch, channel, or streambed.

weight – The weight is the inverse of the variance of an observation.

weights – The set of weights, or the inverse of the variance-covariance matrix of correlated observations.

WGS84 – World Geodetic System. Datum referenced to the WGS 1984 ellipsoid. WGS is the mathematical ellipsoid used by GPS since January 1987. WGS84 is used to identify both a datum and an ellipsoid.

wide-lane – A wide-lane is a linear combination of L1 and L2 carrier phase observations (L1 - L2). This is useful for its low effective wavelength (86.2 cm) and for finding integer ambiguities on long baselines.

working sketch – A drawing prepared from record data depicting the relationship of the various record tracts, usually in, but not limited to, the immediate vicinity of the parcel being considered or surveyed.

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X

X, Y and Z – In the Earth-Centered Cartesian system, X refers to the direction of the coordinate axis running from the system origin to the Greenwich Meridian; Y to the axis running from the origin through the 90× east longitude meridian, and Z to the polar ice cap. In rectangular coordinate systems, X refers to the east-west axis, Y to the north-south axis, and Z to the height axis.

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Y

Y-code – Y-code is an encrypted form of the information contained in the P-code. Satellites transmit Y-code in place of P-code when anti-spoofing is in effect.

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Z

zenith – The zenith is the point at which a line opposite in direction from that of the plumb line (at a given point on the Earth’s surface), meets the celestial sphere.

zenith angle – A zenith angle is the angle measured positively from the observer’s zenith to the object observed.

zenith delay – A zenith is the delay, caused by the troposphere, of a GPS signal observed from a satellite directly overhead. As a satellite approaches the horizon, the signal path through the troposphere becomes longer and the delay increases.

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