Mapping, Platting, and Land Development
Datums, projections, and the plat as a legal document
Know that NAD83 is a horizontal datum and NAVD88 a vertical one, and that GNSS delivers ellipsoidal heights while flood mapping and engineering design use orthometric heights; the difference is the geoid separation, on the order of −80 to −115 ft across the conterminous United States. State plane coordinates are zone-based, and grid distances differ from ground distances by the combined scale factor - multiply ground distance by the factor to get grid, which makes grid distances shorter within a zone's interior.
A subdivision plat is a legal document. Once recorded and lots are conveyed, the plat controls over conflicting deed language. Plan review covers lot area, frontage, setbacks, easements, right-of-way, drainage, and monumentation. Know the difference between dedication of right-of-way and conveyance of fee, and between an ALTA/NSPS Land Title Survey (with its table of optional items) and an ordinary boundary survey.
Contours are interpolated between spot elevations; TIN models and mass-grading calculations follow from them. Accuracy standards attach to a map scale, so a standard quoted without scale is incomplete.
Traps
- Confusing ellipsoid heights with orthometric elevations.
- Applying the scale factor in the wrong direction.
- Treating a FEMA FIRM panel as a survey product rather than a regulatory map.
- Conflating dedication with fee conveyance.
- Citing an accuracy standard without the scale it applies to.
Rule of thumb
National Map Accuracy Standards: 90% of well-defined horizontal points within 1/30 inch at map scale - about ±1.7 ft at 1" = 50'.