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Section 7: GPS Static Surveying

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Overview

Static GPS surveying typically uses a network or multiple baseline approach for positioning. It may consist of multiple receivers, multiple baselines, multiple observational redundancies and multiple sessions. A least squares adjustment of the observations is required. This method provides the highest accuracy achievable and requires the longest observation times; from less than an hour to five hours or longer.

Static positioning is primarily used for ties to the National Spatial Reference System (NSRS) when observing for TxDOT Level 1 and 2 surveys. Project control points are nearly always set using this type of survey.

A variation of the static survey is the faststatic method (also called rapid-static by some manufacturers of GPS equipment). This will allow shorter occupation times (i.e., 8 to 20+ minutes) than static positioning and may use a radial baseline technique, network technique, or a combination of the two. Baseline lengths may not exceed 10 kilometers for L1 only receivers and 20 kilometers for L1/L2 receivers.

Typically, the occupation time is a minimum of 8 minutes for baseline up to 20 km and a minimum of 12 minutes for baselines up to 30 km. Please refer to manufacturers’ specifications for minimum occupation times, number of satellites observed, and minimum amount of cycle slip free data collected for this type of data collection method. FastStatic requires a least squares adjustment or other multiple baseline statistical analysis capable of producing a weighted mean average of the observations. More than one base station will be used to provide redundancy for each vector. FastStatic techniques may be used for observing Levels 3 & 4.

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Planning

Planning is one of the most important parts of the performance of a control survey utilizing GPS survey measurement techniques.

The following steps will help to ensure the creation of a baseline network, which will produce accurate coordinates on newly placed project points:

Anchor: #i1021683Table 3.8 GPS Static Observation Planning

Step

Task

1

Roughly locate both new points and existing control on a map showing roads to use in moving the observers around the project.

2

During field reconnaissance flag and mark points for easy identification by all personnel.

3

For each session draw independent baselines intended for observation on a map. Move through the project until all points have been included.

4

From an almanac of satellite orbits choose appropriate times for observations to avoid consider space weather – unusually poor conditions caused by solar storms and magnetic disturbances can cause many hours of unusable data. One measure of this activity is the Kp index. An explanation of this scale and daily predictions can be found at http://www.sec.noaa.gov/index.html.

5

When possible, separate redundant observations by 24 hours to consider different atmospheric conditions and then a several hour shift to take advantage of a slightly different satellite constellation.

6

Observing the above suggestions, plan your repeated occupations and observations. Make a schedule understandable to all personnel doing the fieldwork.

The field reconnaissance survey of the site mentioned above should accomplish the following:

 

  • 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, notify law enforcement of your 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 multi-path conditions that may affect data collection.


NOTE: All of the control stations selected for reference points must have positions known on the NAD 83 datum. The particular adjustment recommended is the 2003 CORS Adjustment denoted as NAD 83 (CORS).

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Fieldwork

The normal collection rate (epoch) is 5 seconds for static observations but for long observation times of more than 3 hours, a collection rate 30 seconds is acceptable.

Longer baselines will require longer total observation times on end points. Typical observation times are listed in the table below. Allowances should be made for difficult situations where there may be less satellite visibility, high PDOP, chance of reflected signal or even solar flares and sunspots.

Anchor: #i1021714Table 3.9 Typical Static Observation Times

Length of Baseline

Minimum Observation Time *

less than 10 km

45 minutes

10 to 40 km

1 hour

40 to 100 km

2 hour

100 to 200 km

3 hour

more than 200 km

4 hour or more



Minimum observation times for surveys Level 2 and lower.

* Assuming at least 5 satellites and PDOP of less than 6.0.

If adjustable height tripods are used, the height of the antenna above the mark should be measured. It is recommended that this be at a minimum of three locations around the ground plane in two separate units at the beginning of the observing session and again at the end of the observing session.

There are certain newer types of antenna where this is not possible; therefore, the manufacturer’s recommended measurement should be followed. The H.I. must be recorded in a field book or on log sheets for every occupation. The log sheets may contain other information but their main purpose is to pass on to the processor the H.I. of the setup and match the location to the particular data file.

Some antenna setups will require a diagonal (slope) distance to be measured from the edge of the ground plane to the monument. Follow the equipment manufacturer’s instructions and include the type of measurement on the data sheet. For Trimble antennas (with ground plane), the diagonal measurement is made from a notch on the outer perimeter of the antenna and must be noted as such on the log sheet.

There should be one log sheet per observation. At the end of the day or the end of the project, the party chief, knowing the number of observations, must collect all of the completed data sheets. One missing data sheet may require the repeat of an entire session because it is not possible to redo a single missing point since simultaneous occupations must be made.

The elevation mask should not be set at less than 13 degrees – 15 degrees is normally used. Data from satellites lower than that is just about useless for surveying; it is too noisy going through the atmosphere. Anything over 15 degrees may be denying the processor access to useful data that he or she may need in some situations. Usually the processing is done at a cut-off elevation of 15 or 17 degrees.

All observers should be well aware of the scheduled start and stop times for each session and should allow plenty of time to find the monument (which should be well marked and flagged before the day of the observation) and allow enough time to set up the antenna accurately.

In some situations, time can best be utilized by observing the station’s azimuth mark (which is usually about a half mile away and visible from the newly placed station) during the long observation time. The azimuth mark generally only requires about 20 to 30 minutes. However, users will probably not be able to safely leave the station setup unattended, unless it is in a secure area such as a fenced in maintenance yard.

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FastStatic (Rapid Static) Positioning

The method of FastStatic/rapid static positioning requires shorter occupation times than static positioning (i.e. 8 to 20 + minutes) and may use a radial baseline technique, network technique, or a combination of the two. Baseline lengths may not exceed ten (10) kilometers for L1 only receivers and twenty (20) kilometers for L1/L2 receivers.

Accuracy degrades at a predictable rate with this type of survey; therefore, longer baselines may be used when design survey quality is not needed. Please refer to the manufacturer’s specifications for minimum occupation times, number of satellites observed, and minimum amount of cycle slip free data collected for this type of data collection method.

FastStatic requires a least squares adjustment or other multiple baseline statistical analysis capable of producing a weighted mean average of the observations. More than one base station will be used to provide redundancy for each vector.

FastStatic or rapid static techniques could be used for observing Levels 3 & 4 listed in this chapter. It provides baselines that do not exceed the maximum distances stated above in the first paragraph of this subsection.

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Downloading the Data

The Trimble program for transferring the raw data files from the receivers to the laptop or PC is called Data Transfer. Microsoft ActiveSync can also be used. One should never leave a project where considerable travel is required before downloading the files and matching the log sheets.

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Processing

Trimble software for this purpose is called Trimble Geomatics Office and includes the basic program, plus two additional individually licensed add-ons called “Wave Baseline Processor” and “Trimble Adjustment.” When loading the observations into the Processor of TGO, care should be taken that each file includes the correct antenna type, antenna height, and type of measurement. Remember that on CORS and Cooperative CORS stations, the measurement is from the antenna reference point (ARP) – this is also the point of reference for all RINEX files.

The processing will produce a “fixed” or a “float” solution and it could be determined using L1 only or L1/L2. The fixed solution is considered best but for extremely long baselines the float solution may be the only solution available. For very short (5 km and less) baseline, an “L1 Only” solution will probably be the final solution. However, users should hope for L1/L2 (also called iono-free) on baselines longer than approximately 5 or 10 km.

Anchor: #i1021736Table 3.10 Baseline Solution Types

 

Up to 5 km

5 – 10 km

10 – 30 km

30 – 200 km

More than 200 km

Desirable

L1 fixed

L1 fixed or

L1/L2 fixed

L1/L2 fixed

L1/L2 fixed

L1/L2 fixed

Acceptable

L1 only

L1 only

L1 fixed

 

L1/L2 float



The processing will generate several other quality indicators. The RMS error estimate of the vector is a good indicator - usually not more than 15 millimeters. A high ratio of difference between the two closest solutions of a baseline length indicates that the integer was easily established and so the result is more assured. Finally, the reference variance should be close to 1.00 – this is the ratio of the actual amount of error to the amount of error expected (given the accuracy in centering the antenna over the point and measuring the H.I.).

Performing loop closures on selected vectors will make blunders apparent. It may take a few tries to determine which vector is at fault. Just as with a conventional traverse, a ratio of precision or parts per million is the method of checking the closures.

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Troubleshooting Problematic Baseline

A problematic baseline can be defined as a line observed with two carrier-phase GPS receivers, L1 or L1/L2, and the baseline solution does not meet the manufacturer’s specification for quality. In most cases, the problematic baseline was observed with enough satellites for a long enough time period, but the quality indicators show the line to be unacceptable.

The first thoughts may be to re-observe the line. However, this should be the user’s last resort. There are enough tools available in the baseline processing software to allow users to examine the observational information to detect obvious problems.

The following are suggestions on what to look for when troubleshooting problematic baselines:

  • Look at the plot of all satellites during the observing session; there is a plot for each receiver. Software packages differ, but common to most is a plot showing each satellite observed, one below the other.
  • What to look for:
    • When a cycle slip occurs, or there is a loss of lock due to obstructions, there will be a break in the line on the graph for that particular satellite.
    • A short break indicates a cycle slip, a longer break; an obstruction.
    • If too many breaks have occurred, eliminate that satellite and try the baseline solution again. In many cases, this solves the problem.
  • Look at the plot of satellites for both receivers.
    • Was the start and stop time approximately the same, or did one receiver start or stop too early or too late?
    • Start and stop times can be changed to encompass only common observing times and then re-observe the baseline.
  • Satellites with a high signal-to-noise ratio (SNR) can cause problems. In many cases, a high SNR occurs when the satellite is close to the horizon. It is possible to have a satellite low on the horizon for the entire session. In that case, the satellite should be eliminated from the solution, then resolve the baseline.
  • Another way to eliminate high SNR on satellites low to the horizon is to raise the elevation mask for the baseline solution.
  • If the length of the session is short, perhaps too short, try a baseline solution with a shorter epoch than normal.
    • If the default on the baseline solution is thirty (30) seconds, try fifteen (15) seconds. This will increase the number of single, double, and triple differences needed to resolve the baseline.
  • If all the above suggestions fail, resolve the baseline using a more precise ephemeris than was started with.
  • As a last resort, the baseline must be re-observed. Be sure to select a time period different from the original observed time. Look at sky plots and select a time with many satellites and an area free of obstructions.
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Network Baseline

TxDOT recognizes there are arguments for and against the use of dependent (trivial) baselines in the least squares adjustment of a network. TxDOT recommends not using dependent baselines.

For any given multiple receiver session, there are n(n-1)/2 total vectors possible, where n = the number of GPS receivers observing simultaneously. The number of independent vectors is n-1.

Using only the independent baselines:

  • prevents adjusting the same observations more than once and misstating the network degrees of freedom in the least squares adjustment
  • makes it easier to troubleshoot and evaluate the network and locate deviant baselines.
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Accuracy Standards for Network Baseline

For a station to qualify for an accuracy classification, network or local, it must meet the listed accuracy standards, relative to all other stations in the network and/or datum, whether or not there was a direct connection between them.

The table below outlines requirements for network design.

Anchor: #i999719Table 3.11 Minimum TxDOT Network Design Specifications

Level of Accuracy *

Level 0

Level 1

Level 2

Level 3

Minimum Number of Closest Direct CORS Ties

2

2

1

0

Minimum Number of Total FBN/CBN /CORS Station Ties

4

4

3

2**

Minimum Number of Horizontal Station Ties

4

4

(Level 0 ties)

3

(Level 1 or 0)

2

(Level 0,1, or 2)

Minimum Number of Vertical Ties (2nd order or better)

6

5

4

2

Minimum Number of Occupations Per Station

2

2

2

2

Minimum Number of Repeat BL’s (% of all BL’s)

50%

40%

30%

20%

Time Offset Between Observations

(Occupations ***)

± 4 hrs

± 3 hrs

± 2 hrs

± 1 hr

Minimum Satellite Elevation Mask

15 Degrees

15 Degrees

13 Degrees

13 Degrees

Minimum Number of Quadrants for H Station Ties

4

4

3

2

Minimum Number of Quadrants for V Station Ties

4

4

4

2

Type of Ephemeris Required

precise

precise

rapid or precise

broadcast

or better



* Level 4, 5, 6 and 7 surveys are generally not network surveys. Network requirements do not apply.

** These should be at least be indirect ties to CORS, FBN or CBN stations. They may be surveyed from Level 2 stations, which have been directly tied to CORS, FBN or CBN stations

*** To qualify for a new occupation, the observer must remove the GPS receiver at the station and a completely new setup over that station must take place.

FBN and CBN stations are statewide GPS survey networks that form the highest order of monumented control for the NSRS. These are A and B order points. NGS-maintained FBN stations at 100 km station spacing and volunteer-densified CBN points at 25 – 50 km spacing are included in the Table 5.1 and serve as control for regional and local surveys.

Ideally, the time offset between observations should be 24 hours plus 3 – 9 hours before the second observation in order to “see” a completely different satellite constellation. A more practical approach for scheduling observations with a minimum of overlap is to remember that the satellite positions repeat about every 12 hours (actually they advance in position about four minutes a day). Scheduling with this information in mind could result in substantial savings in time and cost. Also, it should be noted that whenever possible, a different receiver should be used at that station for the repeat observation.

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Example of a Network Design Procedure

Network Design Example:

  1. Roughly locate both new points and existing control on a map showing roads to use in moving the observers around the project.
  2. From reconnaissance and mission planning software, determine the best times to observe.
  3. For each session, draw the independent baselines chosen to be observed on map. Move through the project until all points have been observed.
  4. Observing the rules for time differences, plan the repeated occupations and observations. Consider redundancy requirements.
  5. Measure and record antenna height in two different units at the beginning and before the end of each session.
  6. Fill out observation sheet each session.
  7. Every one moves every session (where practical).
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Adjustment

Once vectors have been processed, a least squares adjustment of the network will produce the best possible solution of final coordinates. The first adjustment should hold only one point to known coordinates. The results will indicate how well the GPS derived baselines fit together. If there had been a bad observation, it would show up here as an anomalous vector.

Then after ensuring that only good quality GPS baselines have been produced, the user can proceed holding each known reference station in subsequent iterations of the least squares process. By watching how remaining known points compare, the user will get an idea of how well the control points fit together. At this point, it can be seen how important it is to have additional control points as checks. The user may find that what was thought was a good control point might have to be thrown out.

The user should be aware of standard least squares quality indicators. The final network should pass the chi-square test; the network reference factor should be about 1.00 (plus or minus .10) and the scalar will usually fall between about 5 and 10.

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Minimally Constrained Adjustment

A minimally constrained adjustment (MCA) is an adjustment with only one control point held fixed in the survey network. Holding one control point fixed, shifts observations to the correct location within the chosen datum. Not fixing a control point forces the software to perform a free adjustment. A free adjustment is accomplished by minimizing the size of the coordinate shift throughout the network. This equates to a mean coordinate shift of 0 (zero) in all dimensions.

A minimally constrained or free adjustment acts as one quality control check on the network. This adjustment helps to identify bad observations in the network. If an observation does not fit with the rest of the observations, it is highlighted as an outlier. The minimally constrained or free adjustment also checks on how well the observations hold together as a cohesive unit.

All minimally constrained adjustments must be performed in the WGS-84 datum. Since all GPS observations are made on the WGS-84 datum, the adjustment of the observations should be tied closely to the WGS-84 datum. Realistic error estimates for tribrach centering and H.I. measurement should also be factored into the minimally constrained adjustment.

The following minimally constrained adjustments should be done for Level 1 and Level 2 surveys. The required reports and/or spreadsheets are listed.

An MCA to determine network reference accuracy:

  • Submit a minimally constrained adjustment holding the closest CORS fixed – use the NAD83 CORS coordinate in latitude, longitude, and ellipsoid height.
  • Create a spreadsheet (or select a report) to compare the published CORS coordinates to the coordinates determined in the MCA.

An MCA to determine local HARN relationship if applicable:

  • Submit a minimally constrained adjustment holding the highest order (1st priority) and most central to the project (2nd priority) HARN station.
  • Create a spreadsheet (or select a report) that shows the comparison between the measured values and the published values of other HARN stations included in the survey.

An MCA to show the relationship of bench marks used in the survey:

  • Submit a minimally constrained adjustment holding the highest order (1st priority), highest stability monument (2nd priority), and most central (3rd priority) to the project vertical control stations.
  • Create a spreadsheet (or select a report) showing differences between published orthometric heights (elevations) and measured values.

The minimally constrained adjustment is an iterative process. Perform the minimally constrained adjustment to check the observations for internal consistency and estimates errors for all observations.

If bad observations are found, they should appear as outliers in a histogram of standardized residuals. If bad observations are discovered, they should be removed, one at a time, starting with the largest, so that the statistics of the network are not skewed.

An adjustment should then be performed again. Errors are estimated again. In the subsequent adjustments, the estimated error may be rescaled to produce more realistic error estimates.

These procedures should be repeated until the results meet the following conditions:

  • all outliers have been removed from the network
  • observations have the most accurate error estimates possible; and observations are adjusted such that they fit together well.

During the iteration process, two least squares statistics should be used to gauge progress:

  • Reference factor – The reference factor shows how well the observations, along with their respective error estimates, are working together. Once the reference factor approaches 1.00, the errors in the observations are properly estimated and all observations have received their appropriate adjustments.
  • Chi-square test – Typically when the reference factor approaches 1.00, the chi-square test of network error estimates, degrees of freedom, and level of confidence will pass. At this point, there is confidence that the network observations are working together and that there are no large errors remaining in the network.

Once the minimally constrained adjustment has been completed, move on to the fully constrained adjustment to fit the observations to the local control datum.

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Fully Constrained Adjustment

The fully constrained adjustment (FCA) transforms the network of observations to the control points in the network. Once the network is fixed to those control points, adjusted coordinates based on the project datum (using the appropriate datum adjustment as recommended by TxDOT) for all other points in the network can then be determined.

Use this step to check that the existing control fits together well. The minimally constrained adjustment (MCA) showed that the observations fit together and a fairly rigid network is defined. It is assumed that if any large errors are present in the fully constrained adjustment, the source is non-homogeneous control points (values). Any ill-fitting control points should not be fixed (constrained).

When designing the network, it is good practice to use a minimum of three (3) horizontal control points and four (4) vertical control points because two (2) horizontal and three (3) vertical control points are required to define transformation parameters. The additional horizontal and vertical control points can be used to check the consistency of the adjustment and defined transformation parameters. Adding additional control points builds more confidence in the calculated parameters. Levels 1 and 2 do require these three (3) horizontal coordinates and four (4) elevations at a minimum.

In the fully constrained adjustment, begin fixing the control values to determine how well the rigid network of observations fit the control. Essentially, the adjustment determines if the network of observations fit the network of fixed control points given some error estimate. These error estimates consist of the error estimates along with the applied scalar and set-up errors. The transformation parameters should then be calculated to allow the observations to fit to the control.

The following fully constrained adjustments (FCA) for Level 1 and Level 2 should be delivered along with the listed spreadsheets or reports.

An FCA to determine local accuracy for horizontal positions only:

  • Submit a fully constrained adjustment fixing a minimum of three (3) horizontal stations as noted above.
  • Submit a spreadsheet (or select a report) showing the comparison between the MCA above and the FCA for horizontal position.

An FCA to determine local accuracy for orthometric heights (elevations):

  • If there are unexpected differences in the MCA and published values for vertical, submit a fully constrained adjustment fixing a minimum of 4 bench marks.
  • In many cases, a fully constrained adjustment will not be required for the final elevations of a control survey.
  • If the differences between the published and measured values of the MCA holding one benchmark fixed, fall within the acceptable error limits of a particular level of survey, the MCA elevations will be acceptable as the final results of the survey.
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Determining Elevations

After the fully constrained adjustment, the user will still only have heights measured from the ellipsoid (sometimes called GPS heights). It is necessary to determine orthometric elevations for use in the field. The use of a geoid model, such as GEOID03, will usually yield orthometric elevations accurate to within a few centimeters in many places but for design survey accuracy it will be necessary to hold known elevations surrounding the project to get results within millimeters in relation to the surrounding marks.

From those ties, the geoid model is interpolated throughout the network to produce elevations on the newly surveyed points. Again, it can be seen why it is important to have more than just a couple of control points; vertical control points should be well spaced and surround the project whenever possible.

When processing the data, there are five (5) steps to follow for estimating GPS-derived orthometric heights:

Anchor: #i1279015Table 3.12 Steps in Processing the Data

Step

Action

Perform a 3D minimally-constrained, least squares adjustment of the GPS survey project, i.e., constrain one latitude, one longitude, and one orthometric height value.

Using the results from the adjustment in procedure 1 above, detect and remove all data outliers. The user should repeat procedures 1 and 2 until all data outliers are removed.

Compute differences between the set of GPS-derived orthometric heights from the minimally constrained adjustment (using the latest National geoid model, e.g., GEOID03) from procedure 2 above and the published NAVD 88 bench marks.

Using the results from step 3 of this table, determine which bench marks have valid NAVD 88 height values. This is the most important step of the procedure. Determining which bench marks have valid heights is critical to computing accurate GPS-derived orthometric heights. The user should include a few extra NAVD 88 bench marks in case some are inconsistent, i.e., are not valid NAVD 88 height values.

Using the results from step 4 of this table, perform a fully constrained adjustment holding all valid known values fixed to arrive at the resulting elevations.


The following table provides adjustment analysis information:

Anchor: #i1008888Table 3.13 Office Procedure Guidelines

Adjustment Analysis Criteria

1 cm Horizontal

2 cm Vertical *

2 cm Horizontal

5 cm Vertical

Maximum variance of unit weight (1.0 ideal)

1.5

1.5

Minimum degrees of freedom per station

2 degrees of freedom

1 degree of freedom

Standard deviation of observation residuals, cm

.01 cm

0.1 cm

Standard error of baseline components, cm

.01 cm

0.1 cm

Standardized residuals - pass chi square test

- pass tau criterion

yes

yes

yes

yes

Maximum % observations rejected

10%

10%



*Local Network Accuracy

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Deliverables

The final list of coordinates from the survey should include all metadata necessary to make the coordinates usable. This would include datum (and adjustment such as HARN), units, state plane or surface adjusted, and if surface adjusted, the adjustment factor is used. Do not truncate coordinates. Coordinates shall include standard TxDOT feature codes. Most often the format desired is: point name, northing, easting, elevation, feature code. Occasionally, designers request LandXML.

As an indicator of the survey quality, a project summary should be printed as supplied by the software. It will indicate the above information about the baseline processing and the adjustment routine. Such things as histograms and bell curves, ratios of precision for each point, etc. are generally available for review. The raw data in RINEX or DAT file format and log sheets must also be preserved for future retrieval.

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