system
Given the following post, respond as if you're commenting on Reddit:
human
If polar caps are melting, how do we measure altitude of a place in a precise, consistent and time-invariant manner?
gpt
Altitudes are measured with respect not to the instantaneous sea level at any point in time, but to a datum surface which is periodically refined or replaced. At present, the most widely used datum is the WGS 84 reference ellipsoid.