QUESTION:  
It is said that the MPF will land at Ares Vallis in an area 
below sea level.  I know there is no sea, so how do you determine 
where 0 datum or sea level is on Mars?

ANSWER from Ken Edgett on January 25, 1997:
(quoting text he wrote for the Planetary Society's MARSLINK Kit #2, 
(c) 1994)

Topographic mapping of Mars is very similar to methods used to map 
Earth. The exception is that Mars has no seas or oceans, hence it has
no actual sea level.  How do cartographers determine martian "sea 
level"? Mars scientists refer to the sea level as the *topographic datum*.

This datum is the elevation designated as zero.  For Mars, the zero
elevation is defined by the mean martian radius, 3382.9 kilometers,
and the [average] atmospheric pressure [which] is 6.1 millibars
(6.1 thousandths of the Earth's atmosphere).  If you were standing on
the martian surface and the center of the planet were 3382.9 kilometers
beneath your feet, then you would be standing at 0 kilometers 
elevation. Likewise, if the surface depth to the center of the planet 
were 3407.9 kilometers, you would be at an elevation of 25 kilometers. 
You'd probably be standing on [the summit of] Olympus Mons.

To determine the elevations... scientists combined 1972 Mariner 9
mission data with Earth-based radar.  The ultraviolet spectrometer
instrument on Mariner 9 measured the intensity of ultraviolet light
scattered between the orbiting spacecraft and the surface.  By
analyzing the amount of atmosphere under the Mariner, scientists
measured elevations to an accuracy of 0.6 kilometers.  The higher
the atmospheric pressure, the lower the elevation (when compensated
for temperature).  The lowest elevation is in the huge impact crater,
Hellas Planitia.  This is also the location of the highest
atmospheric pressure on Mars.