RTK Control for a 14-Mile Waterline
Your firm is establishing control for a 14.2-mile rural waterline in Oklahoma. The design is in Oklahoma State Plane North Zone, NAD83(2011), U.S. survey feet, but construction stakes must be set on ground distances. You observe six RTK points with a 25-minute session using a single CORS about 34 km away. Repeated observations of the same six points show horizontal differences up to 0.09 ft and ellipsoid-height differences up to 0.14 ft; one point, BM-17, differs 0.38 ft horizontally from its second observation. The design engineer gives a combined scale factor of 0.99987 for the corridor. A check between two points gives a grid distance of 2,640.000 ft. The client wants 'GPS coordinates' and a one-page control report by Friday.
1. Decide whether the RTK observations are adequate for primary project control and identify the likely blunder. State what additional observations you would require.
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The observations are not adequate as primary control. A 25-minute RTK session at 34 km from a single CORS is marginal for the required repeatability, and the 0.38-ft discrepancy at BM-17 is too large to accept as normal noise. The pattern points to an occupation problem at BM-17, such as multipath, an incorrect antenna height, a bad fix, or a disturbed mark, rather than a network-wide datum problem. I would reject BM-17 until it is reobserved independently, and I would not use the six RTK points as the sole control for a 14-mile project. Require independent static sessions of at least 30 minutes, preferably two sessions on separate days, tied to the published CORS and looped through the project. Check loop closures and compare to published NSRS coordinates. For construction stakeout, establish a few high-quality static control points and use RTK from those, with daily checks on independent points. Use GEOID18 for orthometric heights and verify critical elevations by differential leveling.
2. Compute the ground distance corresponding to the 2,640.000-ft grid distance. Show the formula and explain why the sign/magnitude matters for stakeout.
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The combined factor converts between grid and ground: ground = grid / combined factor. Here ground = 2,640.000 / 0.99987 = 2,640.343 ft, or about 2,640.34 ft. The ground distance is 0.343 ft longer than the grid distance. The sign and magnitude matter because a stakeout crew using grid distances would set points about 0.34 ft short over this interval; over a 14-mile corridor the cumulative error would be substantial if the scale factor were ignored. The correct field procedure is to compute ground distances from grid distances using the project combined factor, or to use a site calibration that properly accounts for scale, and to state clearly on the stakeout list whether distances are grid or ground.
3. List the datum, height, and metadata you would put in the control report, and explain how you would handle BM-17.
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The control report should state the horizontal datum and realization, NAD83(2011) epoch 2010.0, the Oklahoma State Plane North Zone, the units in U.S. survey feet, the projection and combined scale factor used, the vertical datum NAVD88 with GEOID18, and the source CORS and observation dates. It should include the point descriptions, monument types, stability, antenna heights, session lengths, processing software, loop closures, and the final coordinates with their estimated uncertainties. BM-17 should be flagged as rejected or reobserved, not averaged into the network. If it is a critical benchmark, reoccupy it with an independent static session and check the mark for disturbance; if the discrepancy persists, use a different nearby control point and report the problem to the client.