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Section 3: Introduction to Surveying

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Overview

The information found within this section is a product of the former Texas Department of Transportation (TxDOT) Metric Surveying Subcommittee and the Standing Committee on Surveying (SCOS).

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Types of Surveys

Surveying is the science of determining relative positions of points on or near the earth’s surface. The horizontal positions of these points are computed from distances and directions, and vertical positions from differences in elevations which are measured individually or in combination to a specified degree of accuracy, and are in direct relation to a known or determined datum.

Accuracy is a prime consideration in surveying. Instruments for each type of survey are used with prescribed techniques to achieve a designated accuracy.

Less accuracy than specified results in a survey which will prove useless for its intended purpose; more accuracy wastes time and effort, and may not improve the final results.

Surveys are normally divided into two general classes: Geodetic and Plane.

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Geodetic Surveys

The mathematical shape of the earth is an oblate spheroid (almost a sphere) with a major diameter at the equator of about 7,920 miles. Distances or areas measured on the surface of the earth are, therefore, not along straight lines or planes, but are on a curved surface. Geodetic surveys that normally extend over long distances and cover large areas must have computations to allow for curvature of the earth.

To accomplish this, the earth’s major and minor diameters are computed accurately, and from these a spheroid reference. The position of each geodetic station is related to this spheroid. The positions are expressed as latitudes (angles north or south of the equator) and longitudes (angles east and west of the prime meridian), or as plane coordinates on a rectangular grid system, correlated with the latitude and longitude. In addition, the plumb line deflection and its effect on relative positions of the stations are considered in precision work.

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Plane Surveys

When the extent of the survey becomes small (less than 100 square miles in area), and when only limited accuracy is required, the effect of curvature can be ignored. These surveys are treated as if the measurements were made on a plane and are known as plane surveys.

The difference between plane and geodetic surveying can be expressed in terms of plumb lines. In plane surveys, plumb lines are considered parallel, while in geodetic surveys convergence is taken into account.

Highway and railroad surveys, which may extend for hundreds of miles, are usually in a narrow strip and are considered plane surveys. However, a limited computation for the earth’s curvature is necessary in this case. On a long traverse survey, an astronomic azimuth is determined at intervals of several miles.

The astronomic azimuth establishes an astronomic north-south line and may be used to obtain the true direction of a survey line. The azimuth values of the lines between astronomic azimuth stations consider the convergence of the meridians. The methods, operations, and measurements in either type of survey are similar; but since the distances between stations are usually much greater in geodetic surveying, more precise equipment and procedures are used.

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Surveying Field Work

Field work in surveying consists of making and recording measurements. The operations are as follows:

  1. Measuring distances and angles to:
    • establish points and lines of reference for locating details such as boundary lines, roads, buildings, fences, rivers, bridges, and other existing features
    • stake out or locate roads, buildings, utilities, and other construction projects
    • establish lines parallel or at right angles to other lines, measure inaccessible distances as across rivers, extend straight lines beyond obstacles such as buildings and do any work that may require use of geometric or trigonometric principles.
  2. Measuring differences in elevations and determining elevations to:
    • establish permanent points of known elevation (bench marks)
    • determine elevations of terrain along a selected line or area for plotting profiles and computing grade lines
    • stake out grades, cuts, and fills for construction projects.
  3. Making topographic surveys wherein horizontal and vertical measurements are combined.
  4. Recording field notes to provide a permanent record of the field work.
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Planning

Planning is probably the most important part of the performance of a control survey utilizing GPS survey measurement techniques. Proper planning will give one added confidence that quality data will be collected. Regardless of the level of the survey, the items listed below should be addressed before the field data collection process begins.

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Reconnaissance

Prior to the commencement of any TxDOT survey, all significant aspects of the project should be understood so that the project can be performed effectively and efficiently.

For GPS surveying perform a reconnaissance survey of the site to:

  • determine the location and sky visibility of existing and new control stations
  • pick the locations for new stations making sure satellites can be recorded in a minimum of three quadrants
  • look at logistics of project and determine transportation required
  • gain permission to access station(s) on private land
  • if applicable, the surveyor should notify law enforcement of their activities; record sky visibility chart data and access requirements for all stations
  • look for any objects that could be sources for radio interference
  • look for any multipath conditions that may affect data collection.
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Factors Affecting Field Work

The surveyor in the field must constantly be alert to the different conditions he or she encounters and the requirements of the survey. The weather, terrain, personnel, purpose, and accuracy of the survey, systematic procedures, and the expected rate of progress are some of the factors that will affect the work.

Physical factors such as terrain and weather will affect each field survey in varying degrees. Measurements using telescopes can be stopped by fog, mist, or dust. Swamps and flood plains under high water can impede taping surveys. Lengths of light-wave distance measurements are reduced in bright sunlight. Generally, reconnaissance will predetermine the conditions and alert the survey party to the best method to use and the rate of progress to be expected.

The status of training of the personnel is another factor that affects field work. Experience in handling the survey instruments and equipment can shorten survey time without introducing errors, which would require resurvey. The personnel factor is a variable that will affect the rate of progress.

The purpose of the survey will determine the needed accuracy, which, in turn, will influence the selection of instruments and procedures. For instance, comparatively rough procedures can be used in measuring for earth-moving, but grade and alignment of a highway must be much more precise, and require more accurate measurements. Each increase in precision also increases the time required to make the measurement, since greater care and more observations must be taken.

Each survey measurement will be in error to the extent that no measurement is ever exact. Besides errors, survey measurements are susceptible to mistakes or blunders. These arise from misunderstanding the problem, poor judgment, confusion on the part of the surveyor, or simply from an oversight. By working out a systematic procedure, the surveyor will often detect a mistake when some operation seems out of place.

Survey speed is not the result of hurrying; it is the result of saving time through the following:

  • the skill of the surveyor in handling his field equipment
  • the intelligent planning and preparation of the work
  • the process of making only those measurements that are consistent with the accuracy requirements.
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Satellite Health and Availability for GPS Surveying

Only healthy satellites should be observed during the course of data collection. The satellite health situation can be checked by accessing the latest GPS status message from the USCG website at http://www.navcen.uscg.gov/. This status message can also determine if there were problems after the data collection period is over.

There are times of the day when the numbers of satellites available will vary. Especially with real-time kinematic (RTK) positioning planning a work around for these times greatly increases productivity and the quality of results. Most, if not all, GPS software packages include a utility allowing the user to predict satellite coverage. A minimum of five (5) satellites are to be logged for any GPS work. In order to project satellite availability, the software will require a recent ephemeris file.

One Internet site for obtaining this file is: http://www.trimble.com/planningsoftware_ts.asp?Nav=Collection-8425

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Sky Visibility for GPS Surveying

Prior to data collection, the surveyor should look at each station to determine the extent, if any, of sky visibility obstructions greater than ten (10) degrees above the horizon. This survey should include obstructions in all four (4) quadrants of the sky.

If there are obstructions, the most desirable place for those obstructions to be located is northward of the station to be surveyed because of the design of the satellite constellation. If there is an obstruction in that area, it could still be a source of multipath at the GPS antenna. Therefore, the obstruction should be located.

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Satellite Geometry for GPS Surveying

The geometric quality of a constellation of satellites is measured by Position Dilution of Precision (PDOP). It is also measured by Geometric Dilution of Precision (GDOP). The difference between PDOP and GDOP is that GDOP considers time, where PDOP only considers geometry.

The user should be aware of the manufacturer’s recommendations of maximum DOP values for the various types of surveys the user will perform. The vertical component of the GPS position is the most likely component to lack in quality if the DOP values are high. Therefore, if performing a vertical control survey, collect data with conservative DOP values.

One way to ensure that quality data are collected for the vertical is to collect satellite data that includes at least one satellite that is tracked greater than seventy (70) degrees above the horizon. However, a VDOP of less than 4.0 is all that is required. A PDOP of over 6.0 should probably be considered to be too great for usable data, making a PDOP of over 7.0 is unacceptable. Static data during periods of high DOP values should be deleted. Performance of RTK is more demanding and should not be done at PDOP values of 4 or greater.

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Field Notes

The field notes of the surveyor must contain a complete record of all measurements made during the survey with sketches and narration, where necessary, to clarify the notes. The best field survey is of little value if the notes are not complete and clear. They are the only record that is left after the field party leaves the survey site.

All field notes should be lettered legibly. Numerals and decimal points should be legible and permit only one interpretation. Notes must be kept in the regular field notebook and not on scraps of paper for later transcription. The field notebook is a permanently bound book (not loose-leaf) for recording measurements made in the field.

Field note recording takes three general forms: tabulations, sketches, and descriptions. Two, or even all three forms, are combined when necessary to make a complete record.

Tabulation — Measurements may be recorded manually in a field book or they may be recorded electronically through a data collector. Electronic data collection has the advantage of eliminating reading and recording errors.

Sketches — Sketches add much to clarify electronic data collection files and should be used as a supplemental record of the survey. They may be drawn to an approximate scale, or important details may be exaggerated for clarity. Measurements may be placed directly onto the sketch or keyed in some way to the tabular data. A very important requirement of a sketch is legibility. It should be drawn clearly and large enough to be understandable.

Descriptions — Tabulations with or without added sketches can also be supplemented with descriptions. The description may only be one or two words to clarify the recorded measurements, or it may be quite lengthy in order to cover and record pertinent details of the survey.

Note: Erasures are not permitted in field notebooks.

Individual numbers or lines recorded incorrectly shall be lined out and the correct values added. Pages that are to be rejected are crossed out neatly and referenced to the substituted page. This procedure is mandatory since the field notebook is the book of record and it is often used as legal evidence.

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Electronic Data

In nearly all cases, field work is automated by the use of computer software and hardware for collecting, reviewing and editing field measurements. A data collector is connected to the instrument (total station, GPS receiver, digital level, etc.) to store the raw measurement data and perform coordinate geometry (COGO) functions while in the field. Original raw data must be saved as a file for retention as matter of record before any data editing is done.

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Data Collection

It is recommended that field data in electronic form be collected in the AASHTOWare®, Survey Data Management System® (SDMS) Collector and processed in AASHTOWare® SDMS® Processor. This is software developed by AASHTO and supported by TSD. It is provided gratis to TxDOT consultants under TxDOT’s license agreement with AASHTO. Its purpose is to provide a more flexible and user definable method of recording horizontal angles, vertical angles, and slope distances from total stations in a standard format, for use with survey measurement post-processing software.

Radial topographic survey data, traverses, and level runs may all be collected in SDMS® Collector software. The data can then be reduced to coordinates using SDMS® Processor, which uses a least squares type of adjustment. There are a number of useful reports that can be generated in this software.

There are numerous ways to provide connectivity between survey data points. When performing radial topography surveys for a Digital Terrain Model (DTM), points in the same chain such as edge of pavement, centerlines, and ditch lines can be linked together. These survey chains can ultimately be exported to mapping files (2D) or to DTM files (3D) as breaklines. The survey points and breaklines will be used by topographic mapping software to create a Triangular Irregular Network (TIN) and subsequently a DTM. Parcel boundary corners may also be connected with survey chains.

Standard TxDOT feature codes and cells have been developed for use in the field to insure standardization of line weight, color, levels, and symbology. These feature codes also determine whether the points and chains will be exported to a mapping file or a DTM file.

The current list of TxDOT feature codes in Trimble format for TSCe and TSC2 data collectors is the txdotØ6.fcl. This file is available to TxDOT personnel from TSD and consultants may obtain the file from district survey coordinators.

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Survey Review and DTM

Three software programs are authorized by TxDOT to view survey results for troubleshooting and preparation for the delivery of a .dgn file. These programs are Autodesk® CAiCETM Visual® Transportation, Bentley® GEOPAK SurveyTM, and AASHTOWare® SDMS® Processor. This software will accept the SDMS .cal or .pac files as input and, with the TxDOT feature table attached, will graphically display the project for analysis. Corrections and additions can be made and the DTM can then be created. Photogrammetry files, background maps, macros for visualization and other enhancements may be utilized before 2D or 3D graphics are exported as a DGN file for GEOPAK® / Microstation® use by the designer.

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