If the ocean level will rise, how will they measure the correct altitude?

Question

they start measuring from the sea, its 0 meters, right? so if the oceans will rise, wont this confuse the altimeters?

Answer 1

Pressure altimeters are one way of measuring altitude and they have to be continuously calibrated due to varying atmospheric conditions anyway, so it will just require additional calibration.

Answer 2

Most airplanes have radar altimeters --- that bounce a signal off the surface to measure height above sea level.

However the oceans have risen and fallen but hundreds of meters over the ages.

Answer 3

The old fashioned way with surveyed benchmarks.  There are thousands, if not millions, of these throughout the world.