Originally Posted by herm
They take lots of measurements using a radar satellite or GPS to get a Mean Sea Level.
Here is a map of the ocean levels all over the world, yellow is normal, red is higher than it should be and blue lower.. ocean levels are not level all over the world.
That map matches my experience in the south pacific. A couple years ago I revisited my childhood home in Papua New Guinea (island north of Australia). The beach I used to play on with 50+ feet of white sand? Gone. The sand is gone and the trees at the top have died from having their roots exposed. The first house I lived in that was 20' from the shore, that land is now ocean.