Grid vs. Ground: A 2,640-ft Baseline and an 18-ft GIS Shift
A 2,640-ft section-line baseline is measured with GNSS RTK in NAD83(2011), Epoch 2010.00, and reduced to the State Plane Coordinate System. The grid distance is 2,639.66 ft. At the site, the ellipsoid height is 1,050 ft and the geoid undulation is -92 ft. The state plane scale factor at the project is 0.99992. The county GIS parcel fabric was digitized from 1:24,000 USGS quadrangle maps and plots 18 ft east of the GNSS-derived boundary. The client wants the GIS layer shifted to match the survey and asks whether the boundary should be mapped on ground or grid. A pipeline contractor will use the coordinates for a 1,200-ft crossing. The client also wants a shapefile that aligns with the county's parcel layer.
1. Compute the elevation factor, combined factor, and ground distance. Is the 0.34-ft difference significant for the pipeline?
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Elevation factor = R/(R+h) = 20,906,000/(20,906,000+1,050) = 0.9999498. Combined factor = 0.99992 times 0.9999498 = 0.9998698. Ground distance = grid/CF = 2,639.66/0.9998698 = 2,640.00 ft, so the grid-to-ground difference is 0.34 ft. At 1,200 ft, the corresponding difference is about 0.16 ft. For a pipeline crossing with typical tie-in tolerances of plus or minus 0.1 ft, that is significant; the contractor must know whether coordinates are grid or ground. The geoid undulation of -92 ft is not used in the elevation factor; the factor uses ellipsoid height. The computation shows the state plane distortion is small but not zero, and a local ground system or low-distortion projection is appropriate for construction layout.
2. Should the GIS parcel layer be shifted to the survey? Explain the legal and mapping issues.
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Do not shift the county GIS parcel fabric to match one survey. A GIS parcel layer is a graphical and tax representation, not a legal boundary. At 1:24,000, 0.5 mm of map error is about 12 ft, so an 18-ft offset is within the layer's expected uncertainty. Moving the whole layer would corrupt its internal topology and mislead other users. The legal boundary is established by monuments, deeds, and, if disputed, adjudication, not by coordinates or GIS. Keep the survey in its own layer with metadata, and if the client needs visual alignment, use a documented transformation or conflation, not a wholesale edit. Notify the county GIS custodian of the discrepancy; do not alter authoritative data without authority.
3. What coordinate reference system, epoch, and units should be delivered, and what metadata is essential?
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Deliver in NAD83(2011), Epoch 2010.00, State Plane with the correct zone, and state units clearly in U.S. survey feet or meters. Include the projection, zone, datum, epoch, geoid model, combined factor, scale factor, convergence, and whether coordinates are grid or ground. For construction, also provide a local ground system with the transformation to grid. Include control coordinates, GNSS vectors, accuracy estimates, and a statement that the boundary is defined by monuments and deeds. If a shapefile is requested, include a projection file, metadata, and a note that the GIS layer is not authoritative. Do not mix epochs or datum realizations without a transformation.