Geodetic and Control Surveying
Domain 2 tests the reference frames and control procedures behind every coordinate you publish: NAD83(2011) and NAVD88, the Iowa Coordinate System, GNSS positioning, and the reduction of ground measurements to grid.
Reference frames
- Horizontal: NAD83(2011); vertical: NAVD88 (orthometric). Ellipsoid height h and orthometric height H differ by the geoid undulation N (h = H + N); in Iowa N is roughly −31 m.
- NGS is replacing both with the modernized NSRS (NATRF2022, MATRF2022) and a new geoid model; state plane coordinates and existing control will need reprojection, not just relabeling.
- The Iowa Coordinate System uses two Lambert conformal conic zones, Iowa North and Iowa South. Grid distances differ from ground distances; state the combined factor on the plat.
GNSS control
- Static sessions post-processed (OPUS or baseline software) versus RTK corrected by a network such as Iowa DOT's IaRTN.
- Occupy two independent control points, check the published-to-observed residual, and reject a fix with poor residuals, few satellites, or high PDOP. A single RTK shot on one point is not control.
Reduction to grid
Elevation factor = R/(R + h), with R about 20,906,000 ft; combined factor = elevation factor × scale factor; grid = ground × combined factor.
Worked example
Ground distance 5,280.00 ft at h = 1,000 ft. EF = 20,906,000/20,907,000 = 0.9999522. Zone scale factor 0.99993. Combined factor = 0.9998822. Grid distance = 5,280.00 × 0.9998822 = 5,279.38 ft, about 0.62 ft shorter than ground. To go the other way, divide by the combined factor.
Traps
Applying the combined factor in the wrong direction; mixing ellipsoid and orthometric heights; ignoring scale factor near zone limits or at high elevation; orienting from a single point; failing to record the epoch, datum, and adjustment on deliverables.