Very related reading: https://www.groundcontrolpoints.com/
How do geoids relate to datums?
Undulation values represent the distance along the ellipsoidal normal from the ellipsoid to the geoid. As such, each geoid is referenced to a specific ellipsoid and datum. For example, the
Geoid96/Geoid99/Geoid03 undulations are all relative to NAD83, while EGM96 is relative to WGS84. It is important that the correct datum be selected when using a geoid for MSL height input or output since the horizontal position will determine the undulation value applied. Where
the difference between datums is small, such as WGS84 and ITRF, the error will be negligible given the grid spacing of the geoid, but for other datums, such as NAD27 and NAD83, the differences are too large to ignore.
https://kknews.cc/world/lknzeo2.html
https://gisgeography.com/vertical-datum/
http://www.lidarmag.com/PDF/LiDARNewsMagazine_Kessler-ImportanceOfDatumsInMapping_Vol4No5.pdf
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