RFC 7946, "The GeoJSON Format", August 2016

Errata ID: 7261
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: mark hedley
Date Reported: 2022-12-08

Section 4 says:

An OPTIONAL third-position element SHALL be the height in meters above or below the 
WGS84 reference ellipsoid.

It should say:

An OPTIONAL third-position element SHALL be the height in meters above or below the 
EGM2008 geoid vertical datum, the height is defined with respect to the EGM2008 
vertical coordinate reference system (providing close approximation to altitude above 
global mean sea level).

Notes:

Vertical coordinates, where used within GeoJSON shall be interpreted as with respect to the EGM2008 vertical datum. Transformations from WGS84 vertical coordinate values to EGM2008 coordinate values shall not be implemented (unless by prior arrangement between involved parties) .

There is specification information within the EPSG registry on both the EGM2008 vertical datum:
https://epsg.org/crs_3855/EGM2008-height.html
and on the compound of WGS84 latitude longitude with EGM2008 altitude
https://epsg.org/crs_9518/WGS-84-EGM2008-height.html

Common usage of altitude vertical coordinates is with respect to mean sea level. However, the original text within rfc7946 state that a vertical coordinate is with respect to the WGS84 reference ellipsoid.

The WGS84 reference ellipsoid is an oblate spheroid that only partially approximates mean sea level. The EGM2008 (superseding EGM96) vertical datum provides
"Zero-height surface resulting from the application of the EGM2008 geoid model to the WGS 84 ellipsoid." (https://epsg.org/crs_3855/EGM2008-height.htm)
EGM2008 closely models mean sea level as a global vertical datum.

Distortions are global position dependent, discrepancies can range from less than a metre to 10s of metres. For example, at a location of (50.218 -5.327 (WGS84)) the discrepancy between a WGS84 vertical coordinate and a EGM2008 vertical coordinate is 53.4 metres.
This is a really significant vertical discrepancy for a positional coordinate.

The update within this errata matches the specification to the widespread and common use of vertical position with respect to mean sea level, as measured by national mapping agencies and satellite earth observations.

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