Grid, Ground, and the Contractor's File
A contractor is staking a 2,640-ft sanitary sewer extension in the Missouri State Plane East Zone, NAD83(2011), U.S. survey feet. You computed coordinates by OPUS and delivered a point file. The contractor's grader, using a different file from a previous job, sets a manhole 0.42 ft from your staked location. Your field check shows the ground distance between two monuments is 2,640.00 ft by total station; the inverse of your State Plane coordinates is 2,639.58 ft. The contractor says your coordinates are wrong because 'the distance does not match the plan.' The plan shows a ground design distance of 1,250.00 ft between structures. Your client asks for 'coordinates that read ground distance' and wants to use them for construction and for a boundary survey of the easement.
1. Compute the combined factor and explain whether the 0.42-ft difference is an error.
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The combined factor is grid divided by ground: 2,639.58 / 2,640.00 = 0.999841. That is about 159 parts per million. In State Plane, grid distances are generally smaller than ground distances at this location because of projection scale and elevation. The relationship is ground = grid / combined factor and grid = ground x combined factor. For a 1,250.00-ft ground design distance, the grid distance is 1,250.00 x 0.999841 = 1,249.80 ft. The 0.42-ft difference over 2,640 ft is therefore the expected grid-to-ground effect, not evidence that the coordinates are wrong. The contractor's file may use a different zone, datum realization, epoch, units, or site calibration. I would compare a common pair of points using the same metadata and require a check shot before stakeout.
2. What should you deliver for construction stakeout, and what metadata must accompany the file?
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For construction I would deliver either ground coordinates based on stated project control and a documented scale factor, or State Plane coordinates accompanied by the combined factor and explicit instructions for conversion. Machine-control files should be built from a site calibration or localization with checkpoints and residuals, and the contractor should verify two independent monuments before grading. The metadata must include datum and realization, epoch, zone, projection, units, geoid model, combined factor at the site, control coordinates, file format, and the date. The same point file should not be mixed with a file from another project without checking the coordinate system. A note on the plan should state whether distances are ground or grid, because the contractor's 'does not match the plan' complaint is often a units or scale mismatch.
3. Can the same coordinate file be used for the easement boundary survey? Explain the risk.
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The coordinate file can be a reference, but it cannot substitute for a boundary survey. The easement boundary must be based on title research, monument evidence, field measurements, and the boundary of the servient estate. State Plane coordinates are a coordinate system, not a monument; they can be transformed or scaled, but they do not prove where the boundary is. If the easement is described by coordinates, the description must state the datum, epoch, zone, units, and basis of bearing, and it should be tied to physical monuments. A serious risk is using construction coordinates that were never checked against the deed line or existing occupation. I would prepare the easement survey from boundary evidence, show the relationship between grid and ground, and have the legal description reviewed as a title matter.