On Earth we use "sea level" as a reference datum. The elevation of mountains are given in feet above sea level and the depths of the ocean are expressed in feet below sea level. On Mars there is no sea level to serve as a reference. Instead a substitute datum is use. This datum is known as the Mars areoid.
The Mars areoid represents an equipotential surface of the Goddard Mars Gravity Model. The Mars areoid is an imaginary sphere with a center that coincides with the center of Mars and a radius of 3,396,000 meters. We can think of it as a reference elevation, similar to the zero elevation on Earth being mean sea level. (The radius used for the Mars areoid is very close to the average radius of Mars along its equator. That value is 3,396,196 meters.)
To create the Mars topographic map, data from the Mars Orbiter Laser Altimeter as used to calculate the radius of Mars at millions of observation points across surface of the planet. Martian elevation values were obtained by subtracting the radius of the Mars areoid from the radius of Mars at each observation point. The resulting elevations were used to produce the topographic map.
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