This is very important to coordinate systems used to map the height of the ground, because the idea of quantified ‘height’ implies that there is a level surface somewhere below us which has zero height. Even statements about relative height imply extended level surfaces. When we casually say ‘Point A is higher than point B’, what we really mean is ‘The level surface passing through point A, if extended, would pass above point B’ So to accurately quantify the height difference between A and B, we would need to know the shape of the level surface passing through point A. In fact we choose a general ‘reference level surface’ of zero height covering the whole country to which we can refer all our measured heights. This reference level surface is not flat!

— A guide to coordinate systems in Great Britain

I used to think that geo was Hard. But it turns out to be even worse than that.