Grid, Ground, and a State Plane Zone That Won't Hold Still
You are establishing horizontal control for a 3.1-mile gas pipeline corridor that crosses a state plane zone diagonally. Project datum is NAD83(2011), epoch 2010.0. RTN-based RTK vectors give grid coordinates; a total station traverse, reduced to grid using the combined factor at the corridor midpoint of 1.00007, closes 0.31 ft over 16,400 ft. The combined factor at the north end of the corridor is 1.00016 and at the south end 0.99996. A previously published control point at the south end, held fixed at the start of the work, disagrees with your RTK position by 1.4 ft horizontally. The client's civil drawings were prepared in ground coordinates, and the contractor will set pipe from your coordinates using GPS.
1. Compute the consequence of reducing the whole corridor with the single midpoint combined factor, and recommend a coordinate strategy for construction.
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The scale span is the problem. The combined factor ranges from 0.99996 to 1.00016, a span of 0.00020. Using the midpoint value of 1.00007 means that a point at either end is misrepresented by up to about 0.00010 in scale.
0.00010 × 16,400 ft = 1.64 ft
That 1.6 ft is larger than the traverse closure of 0.31 ft, so it is the dominant error in the corridor and the likely source of confusion between grid and ground coordinates.
The traverse itself is acceptable: 16,400 ft closing 0.31 ft is about 1 part in 53,000.
Recommendation: do not simply apply one average factor and then set out with GPS. Define a project ground coordinate system, for example a transverse Mercator with a central meridian through the corridor and a projection height at mean site elevation, or convert each RTK vector to ground with the factor computed at its own location. Publish a control table with state plane grid coordinates, the local combined factor for each point, and the ground coordinates. Give the contractor the ground system and a written transformation between systems, including rotation and translation, so design and stakeout use the same numbers.
2. What should you do about the 1.4 ft disagreement with the published control point before proceeding?
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Do not simply hold the published point, and do not simply discard it. A 1.4 ft disagreement is far outside normal RTK repeatability for a fixed solution on a short baseline, so something is wrong: the mark, the datum/epoch, or the observation.
First, pull the datasheet and check the realization and epoch. The point may be NAD83(1986) or CORS96, or may carry superseded coordinates that were replaced after a readjustment, in which case a transformation, not a scaling, explains the offset. Check the mark's stability and description: a monument in a ditch bank or on a structure may have moved or been reset. Photograph it and compare with the description.
Second, test your own solution: repeat the RTK observation at a different time of day and with a different base or network solution, check the antenna height and the vertical/geoid settings, and review residuals and PDOP. Tie to two other independent marks to see whether the discrepancy is isolated or systematic.
If the mark is disturbed, report it and do not hold it. If it is sound and the offset persists after datum reconciliation, run a short static session on it. Whatever you conclude, document the decision and the evidence.
3. List the metadata that must accompany the delivered coordinates so the contractor and the designer cannot misapply them.
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Coordinates without metadata are a liability. The deliverable should state, at a minimum:
- Datum and realization: NAD83(2011), epoch 2010.0.
- Projection and zone, units (U.S. survey feet or meters), and the false origin.
- Whether the listed values are grid or ground.
- The combined factor at each control point, or the parameters that produce it.
- The transformation between state plane grid and the project ground system: origin, rotation, scale.
- The control source and quality: RTN solution type, repeatability, and static check results.
- A control diagram with ties, and any superseded coordinates.
The contractor setting pipe with GPS will enter the coordinates and a coordinate system definition into the receiver; if the receiver uses grid while the drawings are ground, the pipe goes in 1.6 ft out at the ends, exactly as computed. A note on the drawing that reads coordinates are ground, combined factor 1.00007 is not sufficient either, because the factor varies along the corridor. Publish the per-point values, and issue a brief written explanation of how to check a setout point against the state plane system.