Height measurement on other planets

[Gustav S]

If you measure the height of a mountain, you compare it's height with the sea level. But on let's say mars, how do you measure height when there is no sea to compare with?

For example: Olympus Mons is considered to be the tallest mountain or vulcano in the solar system, but what is the level on which you compare it with? There is no sea level or anything like that.

Thanks

 

[Drakkith]

The choice of the zero mark for elevation is rather arbitrary. For Mars, it is currently determined by an equipotential surface.

From wikipedia: https://en.wikipedia.org/wiki/Geography_of_Mars#Zero_elevation

Zero elevation

On Earth, the zero elevation datum is based on sea level. Since Mars has no oceans and hence no 'sea level', it is convenient to define an arbitrary zero-elevation level or "datum" for mapping the surface. The datum for Mars is arbitrarily defined in terms of a constant atmospheric pressure.

From the Mariner 9 mission up until 2001, this was chosen as 610.5 Pa (6.105 mbar), on the basis that below this pressure liquid water can never be stable (i.e., the triple point of water is at this pressure). This value is only 0.6% of the pressure at sea level on Earth. Note that the choice of this value does not mean that liquid water does exist below this elevation, just that it could were the temperature to exceed 273.16 K (0 degrees C, 32 degrees F).[4]

In 2001, Mars Orbiter Laser Altimeter data led to a new convention of zero elevation defined as the equipotential surface (gravitational plus rotational) whose average value at the equator is equal to the mean radius of the planet.[5]