Ground Versus Grid Coordinates for a Five-Mile Control Network
You are establishing control for a 5-mile boundary of a New Mexico State Trust Land parcel near Roswell. The project datum is NAD83(2011), New Mexico Central Zone (ftUS). Your static GNSS network yields grid coordinates with 0.03-ft relative uncertainty. Between two control points the GNSS grid distance is 5,280.00 ft. A total-station traverse between the same monuments measures 5,281.9 ft after reducing slope and temperature. The average ellipsoid height along the line is 4,920 ft, and the grid scale factor at the project latitude is 0.99987. The client’s GIS vendor delivered a parcel layer with a line 5,279.1 ft long and coordinates rounded to the nearest foot. A technician proposes to “adjust” the GNSS coordinates so the grid distance equals the total-station ground distance, then rotate the parcel to the deed bearings. The plat must close within 0.05 ft and be recorded in Chaves County.
1. Compute the combined factor and expected ground distance. Does the total-station measurement support the GNSS control?
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The combined factor is the grid scale factor times the ellipsoid reduction: CF = 0.99987 × R/(R+h). With R = 20,906,000 ft and h = 4,920 ft, R/(R+h) = 20,906,000/20,910,920 = 0.9997647. CF = 0.99987 × 0.9997647 = 0.9996347. The expected ground distance for a 5,280.00-ft grid distance is 5,280.00/0.9996347 = 5,281.93 ft. The total-station value of 5,281.9 ft differs by only 0.03 ft, which is consistent with the stated GNSS uncertainty. The technician’s proposed adjustment is therefore unnecessary and wrong: it would force a grid distance to equal a ground distance by distorting the control.
2. Should you rotate the parcel to deed bearings and adjust the GNSS coordinates?
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Do not rotate the parcel to deed bearings without establishing the basis of bearing. Deed bearings are record bearings; they may be referenced to true north, magnetic north at the deed date, or an assumed meridian. I would establish the basis of bearing from the GNSS control and a published monument, then compare the deed bearing to the record line. The GNSS coordinates should remain on NAD83(2011) Central Zone. If the project needs ground coordinates, define a low-distortion system using the combined factor and document it; do not alter the geodetic coordinates. The plat should state grid or ground, datum, epoch, units, and combined factor.
3. How should the GIS parcel line be treated in the boundary plat?
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The GIS line is 5,279.1 ft, about 2.8 ft shorter than the expected ground distance of 5,281.9 ft and 0.9 ft shorter than the grid distance. That is too large for the 0.05-ft plat closure and is likely due to rounded coordinates, a different projection, or digitizing. I would not use the GIS layer as control. I would compare its vertices to the GNSS coordinates, transform it to the project datum if needed, and show the record boundary from the deed and control. The plat should close within 0.05 ft on the stated coordinate basis and include a note that the GIS layer was not used for the boundary.