Section 6: GPS Static Surveying
Anchor: #i1028252Overview
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.
Anchor: #i1028279Planning
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:
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Step |
Task |
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1 |
Roughly locate both new points and existing control on a map showing roads to use in moving the observers around the project. |
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2 |
During field reconnaissance flag and mark points for easy identification by all personnel. |
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3 |
For each session draw independent baselines you want to observe on a map. Move through the project until all points have been included. |
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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. |
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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. |
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6 |
Observing the above suggestions, plan your repeated occupations and observations. Make a schedule understandable to all personnel doing the fieldwork. |
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The field reconnaissance survey of the site mentioned above should accomplish the following: |
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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).
Anchor: #i1028300Fieldwork
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.
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Length of Baseline |
Minimum Observation Time * |
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less than 10 km |
45 minutes |
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10 to 40 km |
1 hour |
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40 to 100 km |
2 hour |
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100 to 200 km |
3 hour |
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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, you probably will not be able to safely leave the station setup unattended unless perhaps it is in a fenced in maintenance yard where you feel it is secure.
Anchor: #i1028359Downloading 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.
Anchor: #i1028369Processing
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) baselines an “L1 Only” solution will probably be the final solution but you should hope for L1/L2 (also called iono-free) on baselines longer than about 5 or 10 km. The table below show the base
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Up to 5 km |
5 – 10 km |
10 – 30 km |
30 – 200 km |
more than 200 km |
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Desirable |
L1 fixed |
L1 fixed or L1/L2 fixed |
L1/L2 fixed |
L1/L2 fixed |
L1/L2 fixed |
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Acceptable |
L1 only |
L1 only |
L1 fixed |
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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.
Anchor: #i1028393Adjustment
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, which are beyond the scope of this manual, but outlined step-by-step in the software package. As a guide, however, 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.
Anchor: #i1028413Determining 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.
Anchor: #i1028428Deliverables
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 used. Do not truncate coordinates and 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.