How do I calculate sea level on a planet with no seas?
On one (of many) of the planets that I am planning out, there are no seas. The planet is mainly composed of enormous mountains, with some deserts in between.
There are no seas or large bodies of water anywhere on the planet. Not many people like to live on this desolate planet, but those that brave its harsh condition face a boring and necessary consideration.
They need a method to determine the elevation of certain mountains, but without oceans, it is difficult to determine a standard sea-level.
In a planet without seas or any large bodies of water, how do the habitants determine a standard sea-level?
These people have the technology level of the 1700's of Earth, so no futuristic tech.
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1 answer
Use a geoid.
A geoid is, to quote Wikipedia
the shape that the surface of the oceans would take under the influence of Earth's gravitation and rotation alone, in the absence of other influences such as winds and tides.
It can be expressed in part using spherical harmonics. This might sound overly complicated, but Laplace had developed the basics of spherical harmonics in the late 18th century.
Collecting data to calculate the precise shape may be difficult, but so would any other method of gathering enough data to use as a reference point.
An even simpler metric is the reference ellipsoid, but a geoid is closer to actual sea level.
A colorful picture to represent a geoid (because why not?):
Image in the public domain.
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