Hypsometry is the measurement of land elevation
The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....

 relative to sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

. Bathymetry
Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...

 is the underwater
Underwater is a term describing the realm below the surface of water where the water exists in a natural feature such as an ocean, sea, lake, pond, or river. Three quarters of the planet Earth is covered by water...

 equivalent. A hypsometer
A hypsometer is an instrument for measuring height or altitude. Many different physical principles may be used.-Scale hypsometer:A simple scale hypsometer allows the height of a building or tree to be measured by sighting across a ruler to the base and top of the object being measured, when the...

 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 and negative (underwater) values, and are bimodal due to the contrast between the continents and oceans. On other planets within this solar system, elevations are typically unimodal, due to the lack of oceans on those bodies.
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