Hypsometry

Hypsometry (from Greek , hupsos, "height" and , metron, "measure") is the measurement of land elevation relative to sea level.Bathymetry is the underwater equivalent. A hypsometer is an instrument used in hypsometry, which estimates the elevation by boiling water – water boils at different temperatures depending on the air pressure, and thus altitude.

On Earth, the elevations can take on either positive or negative (below sea level) values, and are bimodal due to the contrast between the continents and oceans. On other planets within this solar system, elevations are typically unimodal, owing to the lack of oceans on those bodies.

...
Wikipedia