The zero point of elevation on Mars, the datum, is the elevation at which the atmosphere pressure is 6.1 millibars, or 610 Pascals. Atmosphere pressure has to be used because Mars has no ocean, and we can't use "sea level" like on Earth. More formally, the datum is a fourth-order, fourth-degree surface of equal gravitational potential (determined from the Viking orbiter spacecraft) such that the pressure of the atmosphere is 6.1 millibars. (determined by radio occultations).