GNSS Control Residuals and Grid-to-Ground Scale on a 120-Acre Subdivision
You are establishing control for a 120-acre subdivision in Colorado Central State Plane, NAD83(2011) 2010.0, U.S. survey feet. Six static GNSS receivers observe four new control points in two 45-minute sessions. The vectors agree internally: the worst repeatability is 0.018 ft horizontal and 0.031 ft vertical. When you minimally constrain the network to two NGS monuments, C1 and C2, the residuals at the new points are 0.12-0.18 ft, with C2 showing the largest residual. C2's published coordinate is from 1996 and its monument is a 3.5-inch aluminum disk in a curb that has been struck by a snowplow; the disk is tight but the curb is cracked. C1 is a stainless steel rod with a collar, stable. The project's combined factor at the site is 0.999913. The design engineer's plans use State Plane grid coordinates, and a 2,000.000-ft building dimension scales 0.174 ft shorter on the ground. The contractor is laying out from the grid coordinates and reports a 0.17-ft misfit at the far corner.
1. How should you treat C2 in the adjustment, and what is the technical basis for your decision?
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Do not hold C2 as a fixed, errorless control point merely because it is published. The internal GNSS repeatability is about 0.018 ft horizontal, while the constrained residuals are 0.12-0.18 ft, an order of magnitude larger. That pattern points to control inconsistency, not to a bad GNSS session. C2's description and the cracked curb show a credible disturbance mechanism; the disk can be tight while the curb has rotated or settled. Run a free adjustment and a minimally constrained adjustment on C1 alone, then compare C2 as a check. If C1 is stable and the network agrees with it at the 0.02-0.03-ft level, hold C1 and do not constrain C2. Document the decision, the residual at C2, and the monument condition. If another stable NGS monument is available, include it to strengthen the check. Never average a suspect coordinate into the network just to make the residuals smaller.
2. Compute the ground distance for the 2,000.000-ft grid dimension and explain why the contractor's 0.17-ft misfit is not a blunder.
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Grid distance = ground distance x combined factor. Therefore ground = grid / combined factor = 2,000.000 / 0.999913 = 2,000.174 ft. The combined factor is less than 1 because the State Plane scale factor and the elevation factor both reduce grid distance relative to ground at this elevation. A 2,000.000-ft grid dimension is therefore about 2,000.174 ft on the ground. If the contractor sets out using grid coordinates and then measures between the stakes with a tape or total station on the ground, a 0.17-ft difference at the far corner is exactly the expected grid-to-ground effect, not a 0.17-ft layout blunder. The issue is a coordinate-system mismatch. Either provide layout coordinates that are scaled to ground, or give the contractor the combined factor and require grid-based checks. Do not distort the State Plane coordinates to hide the difference.
3. What deliverables and metadata should accompany the control to prevent this problem on the construction site?
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Deliver a control diagram with the adjusted coordinates, epoch, datum, projection and zone, units, and the combined factor. Include monument descriptions, stability notes, the rejected C2 and why, and the residuals. State clearly whether coordinates are grid or ground. If the project uses a ground-based project datum, publish the transformation or scale factor to State Plane and require all layout to use one system. Provide a benchmark and control check sheet and require the contractor to check into two independent control points before layout. Include a note that the 2,000.000-ft grid dimension is 2,000.174 ft on the ground. Keep the raw GNSS observations and the adjustment report. This metadata is part of the survey record and should be transmitted in writing, not just by word of mouth.