GPS height to elevation (and back)

GPS measures height above the ellipsoid, a smooth egg shape that stands in for Earth. Elevation (what maps and plans use) is height above the geoid, which is close to mean sea level. In most of the lower 48, the geoid is about 80 to 120 ft below the ellipsoid, so your GPS height is smaller than your elevation.

The rule is simple: elevation = GPS height − geoid height. In Pennsylvania the geoid height is about −111 ft, so a GPS height of 1,000 ft is an elevation of about 1,111 ft.

Sources: NGS GEOID18 through the NGS Geoid Height Service API; NGS: NAVD 88.

In the lower 48 and Alaska, the answer is in NAVD 88, the official US elevation datum. In Puerto Rico it is PRVD02, and in the US Virgin Islands it is VIVD09. Lower 48, Puerto Rico and the Virgin Islands use GEOID18. Alaska uses GEOID12B, NGS's latest hybrid model there.

Hawaii has no NAVD 88, so this tool does not give elevations there.

NGS plans to replace NAVD 88 with a new system (NAPGD2022). When that happens, elevations will change by up to a few feet. Today, NAVD 88 is still the official one.

Sources: NGS GEOID18 through the NGS Geoid Height Service API; NGS: New Datums.

A drone or phone with no RTK gives WGS 84 heights. GEOID18 is built for NAD 83(2011) heights. If you pick WGS 84 above, we first shift your point to NAD 83(2011) with EPSG:8970, "ITRF2014 to NAD83(2011) (1)", NGS's published 14-parameter transformation, run in your browser for today's date. That changes the height by about 1 to 5 ft in most of the US.

Without RTK, a raw GPS height can be off by 10 ft or more anyway, so treat that answer as rough.

Heights here use the international foot by default (the one on every tape measure). Over 1,000 ft, the US survey foot differs by only 0.002 ft, so for heights it almost never matters. See the unit converter.