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.