[manarth]

  Even a negative altitude isn't necessarily incorrect,
  if the "altitude" is based on a mean ground level.

This has been niggling at me, so I had to check :-)

As Mars lacks water, the idea of a "zero datum" isn't really meaningful, so an arbitrary point has been used.

Between 1971 and 2001, the "zero elevation" point was an atmospheric pressure of 6.105mb [1] (for comparison, Earth's zero datum is 1013.25mb).

Since 2001, the zero datum definition has been based on the mean radius of the planet (an ellipsoid called the Areoid, similar to Earth's Geoid).

The lowest point on Mars is therefore at an altitude of -8200m (the Hellas impact crater). [2]

(edit) Interestingly, the intended landing site - Meridiani Planum - is "below ground" at an altitude of -250m. [3]

[1] https://en.wikipedia.org/wiki/Geography_of_Mars#Zero_elevati...

[2] http://geology.com/articles/highest-point-on-mars.shtml

[3] http://io9.gizmodo.com/this-elevation-map-of-mars-makes-the-...

[andai]

Thanks for the info.

On a tangentially related note, do you have any idea what happens to the concept of sea level on earth as the sea level changes?

[manarth]

Thank you, and that's a really interesting question.

There are really two concepts in there: the idea of "sea level", and the idea of "altitude", which we generally understand to be height above or below sea-level, but in some circumstances is only tangentially related to sea-level.

Most current geospatial representations use the WGS84 projection, which is the projection used by the original USA GPS system called Navstar.

WGS does get revised every now and again. The last revision was in 2004. However, the concept of WGS is built around an imaginary idealised ellipsoid that represents the shape of the Earth's surface and the Earth's gravitational field, so a change in sea-level wouldn't necessarily change the definition of WGS, and if it were to be updated, it would be slow to update.

That said, GPS is notoriously bad at reporting altitude, so tools tend not to rely on GPS for altitude measurements anyway.

A good example is in aviation. Altitude isn't measured using GPS, it's measured using barometric pressure against one of three references: either the recorded barometric pressure at a particular airfield (QFE), the corresponding barometric pressure at sea-level (QNH) or a global idealised standard barometric pressure (1013mb). During take-off or landing, the altimeter is calibrated to the airfield's barometric pressure, to accurately report the altitude above the airfield. During transition to cruising altitude, many aircraft use QNH. During general flight, altimeters are calibrated to the global standard of 1013mb, so that each aircraft maintains consistent vertical separation. Commercial aircraft flying at "Flight Level" use that standard barometric pressure to calculate altitude. For that use-case, it doesn't matter whether the reported altitude is the height above the ground, the height above mean sea-level, or the height above some arbitrary reference point: what's important is that everyone is communicating using the same reference, which is why the mean atmospheric pressure of 1013mb was adopted.

If (when?) sea-level rises, then small changes will be seen. Nautical charts will be updated with new high-water/low-water marks, maps will eventually be updated, etc. Widely-used references such as WGS84 will take a long time to update, and may not even be updated at all (because an altitude difference of 1 or 2 meters isn't hugely significant). Other reference points, such as the barometric pressure of 1013mb are unlikely to change, partly because they have a long legacy, and partly because the accuracy of the value is to a large part irrelevant.