Section 2: Design Features
Anchor: #i1000160Control
To meet a network or local accuracy level, a GPS project must be connected to sufficiently accurate and well-distributed existing control.
All of the control stations to which the network will be constrained must have positions known on the NAD83 datum. Control stations of the state HARN adjustment are generally used; however, certain special projects may have a legitimate need for another geodetic reference. Use the appropriate datum adjustment as recommended by the TxDOT district surveyor or survey coordinator.
The minimum number of horizontal and vertical constraints is stated in Table 5.1, with their location being distributed in different quadrants relative to the center of the project. Where existing NGS or TxDOT horizontal and/or vertical control on a common datum and epoch is available, all such stations lying within a few kilometers of the survey’s boundaries should, if possible, be included in the survey if they meet the horizontal accuracy requirements. Second order or better is generally required for vertical.
Anchor: #i1000183Orthometric Heigth
Requirements for orthometric height constraints are dependent upon geoid slope, project extent, desired accuracy, and the density of the gravity database. These issues are addressed under the subsection, Orthometric Height Determination in Chapter 6, Section 7.
In general, vertical control for Level 1 and Level 2 networks require a minimum of 4, preferably 5 published vertical control stations. They should be situated on the outside corners of the project at a minimum.
In other words, at least one bench mark should be fixed in each of the four (4) quadrants of the survey area, such that nearly all of the newly surveyed stations will fall inside a boundary drawn around the outside benchmarks. Additional benchmarks inside the perimeter will aid in strengthening the adjustment.
Anchor: #i1000207Network Baseline
TxDOT recognizes there are arguments for and against the use of dependent (trivial) baselines in 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.
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.
|
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.
As mentioned in the Chapter 3 of this manual, 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 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.
Anchor: #i1000268Example of a Network Design Procedure
Network Design Example:
- Roughly locate both new points and existing control on a map showing roads to use in moving the observers around the project.
- From reconnaissance and mission planning software, determine the best times to observe.
- For each session, draw the independent baselines chosen to be observed on map. Move through the project until all points have been observed.
- Observing the rules for time differences, plan the repeated occupations and observations. Consider redundancy requirements.
- Measure and record antenna height in two different units at the beginning and before the end of each session.
- Fill out observation sheet each session.
- Every one moves every session (where practical).