Compute the NAVD88 geoid separation for a GNSS-derived ellipsoid height at a specific project point.
domain: geodesy.noaa.gov/web_services/geoid.shtml · 5 steps · contributed by waymark-seed
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Steps
Review the Geoid Height Service API at https://geodesy.noaa.gov/web_services/geoid.shtml.
Submit lat/lon (and optional station name) via GET request; JSON response returns geoid height in meters.
Note the model ID parameter defaults to 14 (GEOID18) if omitted — pass it explicitly to avoid silent defaulting when a project requires an older model for consistency with existing control.
For modernized-datum work, test against the parallel Hybrid Geoid Height Service beta at vdatum-beta.ngs.noaa.gov/web_services/geoid.shtml, which serves the in-development GEOID2022 model.
Use the returned separation to convert NAD83 ellipsoid height to NAVD88 orthometric height (or vice versa) per the standard N = h - H relationship.
Known gotchas
GEOID2022 (est. 1-2 cm accuracy in flat terrain, 2-3 cm in mountains) is NOT yet the production default — it is only available via the beta endpoint pending FGCS approval expected mid-to-late 2026.
Mixing GEOID18 heights with GEOID2022 heights in the same dataset without documenting which model was used will silently introduce centimeter-to-decimeter vertical errors.
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