The canonical “zero reference point” should be the geoid, but that also changes with time (https://en.wikipedia.org/wiki/Geoid#Temporal_change https://en.wikipe

The canonical “zero reference point” should be the geoid, but that also changes with time (https://en.wikipedia.org/wiki/Geoid#Temporal_change https://en.wikipedia.org/wiki/Geoid#Temporal_change), so yes, it’s not trivial.