Subject: How ca we measure the height of mountains on other planets?

On earth we use the sea level as a basic reference of the height of 
mountains. The height of the mountain on earth is calculated somehow based 
on the weight of the mountain producing something called a geoid(its 
called that in swedish at least). This has the effect that the calculated 
sealevel can be below the actual sealevel. But the real issue is... how 
can we know the height of the mountains on other planets? Olympus Mons on 
Mars i supposed to be much higher then Everest, measuring from what? there 
is no sea that we know of. You cant measure from the lowest point of the 
planet, since that would make everest close to 20000meters. So what is the 
reference point for extraterrestrial mountains?

Re: How ca we measure the height of mountains on other planets?