I wonder what the reference point is (for 0 ft elevation) for Martian elevations. Mt. Whitney is 14,500 ft above sea level, but is only about half that above the surrounding plain.
If Mt. Sharp is 3.4 miles above the surrounding plain, then that is a really tall mountain.
Interesting question...so I looked it up. According to Wiki:
"Since Mars has no oceans and hence no 'sea level', it is convenient to define an arbitrary zero-elevation level or "datum" for mapping the surface. The datum for Mars is arbitrarily defined in terms of a constant atmospheric pressure. During the Mariner 9 mission, this was chosen as 610.5 Pa (6.105 mbar), on the basis that below this pressure liquid water can never be stable (i.e., the triple point of water is at this pressure). This value is only 0.6% of the pressure at sea level on Earth, which forms the zero elevation datum for our planet. Note that the choice of this value does not mean that liquid water does exist below this elevation, just that it could were the temperature to exceed 273.16 K."
It doesn't really answer your question about Mt. Sharp (i.e., how far does it rise above the surrounding plain?), but it does make sense as to how it is calculated. Would love to see a Mars topo map depicting the datum contour for 0'.