[Gmo]

I don't know how altitude is calculated on Mars, but, for instance, there are some landing strips of Schiphol Airport (Amsterdam) that are below sea level and I expect an altimeter to give a negative value there. But this value is correct though !

Not everything is as easy as it seems.

[manarth]

There are two known ways of calculating altitude on Mars: firstly by atmospheric pressure, with an arbitrary value of 6.105mb chosen (based on readings from Voyager). In 2001, a new definition was chosen, based around an ellipsoid (the Areoid) formed by imaginary sphere of the mean radius of the planet.

The effect is that the intended landing site is at an altitude of -250m, and the lowest point on Mars is at an altitude of -8200m.