[doakes]

I work in this space of measuring (mostly coastal) water levels and it's pretty amazing how many different vertical datums there are. When people say something has a height of X, you don't always consider what it's relative to. Is it sea level? High tide or low tide? Maybe it's relative to one of the many geodetic ellipsoids. Maybe a nearby physical benchmark put into the ground by a surveyor. Many cases (like this article) just care about the relative changes locally, but even for that you have to be careful of places like Southeast Alaska where glacial melt causes the land to rise and give the appearance of sea levels dropping.