GPS Control That Won't Close
You are establishing control for a 2-mile boundary survey in rural Alabama. You observe six points with static GPS, 45 minutes each, using a single-frequency receiver and a 30-minute baseline to a CORS station 28 km away. Your loop closure between three points is 0.14 ft horizontal and 0.31 ft vertical. The project datum is NAD83(2011) and the survey must tie to the Alabama State Plane Coordinate System, East Zone. The terrain is rolling, and you plan to use the control for RTK later. The client wants coordinates on the plat.
1. Evaluate whether this control is adequate and what you would change.
Read the worked answer
A 0.14 ft (≈4 cm) horizontal loop closure over 28 km baselines with a single-frequency receiver and 45-minute sessions is marginal - single-frequency receivers cannot resolve ionospheric error over long baselines well, and 28 km is long for L1-only. The 0.31 ft vertical closure is poor and reflects weak geometry and the single-frequency limitation. For a boundary survey where the plat will carry coordinates, you need better: use dual-frequency receivers, shorten baselines (occupy an intermediate point or use a closer CORS), lengthen sessions to 1–2 hours, and observe a closed loop including a known point. Reject the current control for coordinate purposes until it closes to a tolerance you can defend (e.g., 0.05 ft horizontal, 0.10 ft vertical, or per your project spec).
2. What must you do to convert GPS-derived ellipsoid heights to usable elevations, and how do you handle the State Plane coordinates?
Read the worked answer
GPS gives ellipsoid heights; to get orthometric elevations you must apply a geoid model (e.g., GEOID12B or current) - do not treat ellipsoid height as elevation. For State Plane, project the NAD83(2011) lat/long through the Alabama East Zone projection (Lambert Conformal Conic) and apply the combined factor (grid scale × elevation factor) when reducing ground distances to grid. Show on the plat: datum, epoch, geoid model, projection, zone, combined factor, and the control points used. If the plat shows coordinates, label them as State Plane Alabama East Zone, NAD83(2011), and state the units and the combined factor so a retracement surveyor can reproduce them.