# Criteria for the datum

**URL:** <https://boards.straightdope.com/t/criteria-for-the-datum/184907>\
**Category:** Factual Questions\
**Created:** [June 28, 2003, 2:46am UTC](https://boards.straightdope.com/t/criteria-for-the-datum/184907 "2003-06-28T02:46:55Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![raisinbread](https://avatars.discourse-cdn.com/v4/letter/r/8491ac/32.png) [@raisinbread](https://boards.straightdope.com/u/raisinbread)\
**Post date:** [June 28, 2003, 2:46am UTC](https://boards.straightdope.com/t/criteria-for-the-datum/184907/1 "2003-06-28T02:46:55Z")

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I recently started re-reading the Mars trilogy by Kim Stanley Robinson and one term keeps popping up as the analog to Earth’s sea level. Since Mars has no ocean how is this point ascertained and what criteria is used for finding the datum? I doubt this is an arbitrary measurement.

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**Author:** ![Ice\_Wolf](https://avatars.discourse-cdn.com/v4/letter/i/dfb087/32.png) [@Ice\_Wolf](https://boards.straightdope.com/u/Ice_Wolf)\
**Post date:** [June 28, 2003, 3:20am UTC](https://boards.straightdope.com/t/criteria-for-the-datum/184907/2 "2003-06-28T03:20:26Z")

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[How do you determine where 0 datum or sea level is on Mars?](http://www.quest.arc.nasa.gov/mars/ask/terrain-geo/Determining_0_DATUM_or_Sea_Level_on_Mars.txt)

> [@](#):
>
> 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).

Plus, a [nifty map.](http://bulk.vttoth.com/BULK/MOLA/mola.html)