Mapping, Geodesy, and Geographic Information Systems
Core concepts
Candidates must understand the ellipsoid, geoid, orthometric height, ellipsoid height, horizontal and vertical datums, map projections, State Plane coordinates, scale factors, and grid-to-ground conversions. Know that State Plane coordinates are zone-specific and datum-specific; never mix NAD27 and NAD83 coordinates without transformation. GIS concepts include vector and raster data, topology, metadata, accuracy standards, and the difference between a map display and a legal record. Know that combined factor equals grid scale factor times elevation factor.
Traps
A frequent trap is confusing ellipsoid height with orthometric elevation. Another is applying a combined factor in the wrong direction: grid distance equals ground distance times the combined factor, so ground distance equals grid distance divided by the combined factor. Watch for datum shifts, zone mixing, and treating a GIS layer as survey-grade evidence without checking its accuracy and metadata.
Worked example
If the combined factor is 0.99995 and the ground distance is 1,000.00 ft, the grid distance is 1,000.00 × 0.99995 = 999.95 ft. Conversely, a grid distance of 999.95 ft converts to about 1,000.00 ft on the ground. Rule of thumb: the elevation factor changes by roughly 1 ppm for every 21 ft of elevation above the ellipsoid.