12.2 GNSS Positioning in Mississippi: Rule 7.5(2), the GCGC RTN, CORS & Quality Control
Key Takeaways
Rule 7.5(2) requires surveyors using GPS or other radial methods to apply acceptable procedures and adequate quality control so boundary points stay within allowable positional tolerances.
Mississippi's statewide real-time network is operated by the University of Southern Mississippi's Gulf Coast Geospatial Center, which reports 52 CORS across the state.
GNSS measures ellipsoid height h, and orthometric height H is found from H = h − N, where N is the geoid height from the current NGS geoid model.
Redundant GNSS observations are independent only if they are separated in time so satellite geometry and atmospheric conditions change.
Coordinates recorded on a Mississippi plat must come from a connection to a third-order Class I or higher NGRS station (§ 89-6-11).
What the Mississippi rules actually require
| Source | Requirement |
|---|---|
| Rule 7.5(2) | When GPS or other radial methods locate or establish boundary points, the PS "shall apply acceptable surveying procedures and perform adequate quality control measures to ensure that the allowable positional accuracy and/or positional tolerance of such points are not exceeded" |
| Rule 7.5(1) table | Distance (0.03/0.05/0.07 ft + 50/100/150 ppm) and bearing (±1/±2/±3 min) tolerances still apply to the results (Section 3.4) |
| Rule 7.3, item 18 | If State Plane Coordinates are used: zone, datum, method (GPS or conventional) including published control used, combined factor, convergence angle, and coordinates of two reference points |
| § 89-6-11 | Recorded coordinates must come from an accurate connection to a third-order Class I (1:10,000) or higher NGRS station under FGCC standards |
| § 89-6-13 | Coordinate descriptions must be tied to the existing land system and meet the Board's standards of practice |
Beyond those, the rules set no PDOP threshold, epoch count, or re-occupation interval. Statements such as "Chapter 7 requires PDOP ≤ 3.0" or "the Board requires a four-hour re-occupation" are not in the rule. What the rule requires is that the surveyor can demonstrate the positional tolerance was met. The practices below are how surveyors do that; they come from National Geodetic Survey guidance and professional practice.
GNSS methods
| Method | How it works | Typical use |
|---|---|---|
| Static | Two or more receivers log simultaneously; baselines are post-processed | Primary project control; ties to CORS; long baselines |
| Rapid (fast) static | Shorter sessions over short baselines, post-processed | Secondary control |
| Single-base RTK | A base on a known point radios corrections to a rover | Boundary, topography, and layout near the base |
| Network RTK (RTN) | A server models errors from many CORS and streams corrections (for example, through NTRIP) | Boundary and topography statewide where cell coverage allows |
| OPUS | NGS's Online Positioning User Service processes a static file against CORS | Checking or establishing control positions on NAD 83 |
Single-base RTK error grows with distance from the base, and a disturbed base tripod corrupts everything after it. Network RTK reduces the distance-dependent error but depends on a cell connection and on the network's datum realization.
Mississippi's networks
- GCGC Real Time Network. The University of Southern Mississippi's Gulf Coast Geospatial Center operates the statewide RTN. It grew out of the Mississippi Height Modernization program, whose initial CORS network was installed in south Mississippi in 2005, and the Center describes it as 52 continuously operating reference stations throughout the state. It provides NTRIP mountpoints and published station information. Check the network's current datum realization and geoid model before each project.
- NOAA CORS Network (NCN). NGS manages the national CORS network. Data are free and are the basis of OPUS and of the NAD 83 realization on which Mississippi control rests.
Heights
GNSS measures ellipsoid height (h). Mississippi elevations are orthometric heights (H) on NAVD 88:
H = h − N
N is the geoid height from the current NGS hybrid geoid model (GEOID18 for NAD 83(2011)). N is negative in Mississippi, so H is larger than h. Rule 7.5's elevation tolerances for water- and contour-controlled boundaries (±0.05/0.10/0.20 ft) are demanding for GNSS alone. Tidal and lake boundaries usually need leveling from benchmarks or careful GNSS with local checks (Section 10.3).
A defensible quality-control routine
These are widely used practices, drawn from NGS real-time GNSS guidance and professional standards. They are not codified Mississippi numbers.
- Know your datum. Record the network mountpoint's realization (for example, NAD 83(2011) epoch 2010.00) and the geoid model. Don't mix realizations without transformation.
- Check in and check out. Occupy published control or recovered project monuments before and after the session. Compare with published values to catch antenna-height, datum, and setup blunders.
- Fixed solutions only. Never record a float solution for a boundary point.
- Watch geometry and environment. Monitor PDOP and satellite count, use an elevation mask (commonly about 10°–15°), and be wary of canopy and multipath from metal roofs, fences, and vehicles.
- Redundancy that is actually independent. Re-observe boundary points after the satellite geometry has changed substantially, typically hours apart, with a fresh initialization. Compare the positions against the Rule 7.5 tolerance for the class. Two shots a minute apart share the same geometry and errors.
- Tie the survey together. Measure between boundary points conventionally where practical, and connect to at least two control points so rotation and scale are verified. Rule 7.3 requires two reference points whenever State Plane Coordinates are used.
- Keep the records. Keep raw data, residuals, check shots, and the reasoning for accepted positions. They show that the "adequate quality control" Rule 7.5(2) demands was performed.
Common errors
- Antenna-height blunders (meters versus feet; wrong rod height). These corrupt heights and, through the elevation factor, can slightly affect grid distances. Check-ins catch them.
- Wrong mountpoint or geoid. An unexpected constant shift at a known point is the clue.
- Treating RTN coordinates as boundary evidence. A GNSS position of a found monument measures where it is. Whether it is the corner is a question of evidence (Chapters 6–8), not of precision.
- Mixing grid and ground. State on the plat whether distances are grid or ground, and give the combined factor (Section 12.1).
Which statement accurately describes what Mississippi's Standards of Practice require for GNSS boundary work?
Chapter 7 requires PDOP of 3.0 or less and two occupations at least four hours apart
GNSS positions are exempt from the Rule 7.5 tolerances
The PS must apply acceptable procedures and adequate quality control so boundary points stay within the allowable positional tolerance (Rule 7.5(2))
GNSS may be used only for control surveys, never for boundary corners
A rover reports an ellipsoid height of 142.60 ft at a point where the NGS geoid model gives N = −94.10 ft. What is the NAVD 88 orthometric height?
236.70 ft
48.50 ft
142.60 ft
−48.50 ft
A crew observes a corner by network RTK, immediately re-initializes, and observes it again 60 seconds later. The two positions agree within 0.01 ft. Why is this weak evidence of accuracy?
Because RTK can never be used for boundary corners in Mississippi
Because the two positions should differ by exactly the combined factor
Because Rule 7.3 requires four observations of every corner
Because the observations share nearly the same satellite geometry and atmospheric conditions, so they are not independent
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