Altitude on maps is given using sea level as a baseline.
But sea level is not a constant, NASA/JPL oceanographer Josh Willis explains: “Even though it’s sometimes convenient to think of the ocean as a great big bathtub, where turning on the tap at one end raises the water level in the whole tub, real sea level rise doesn’t quite happen that way. To understand why, you first have to realize that ‘sea level’ isn’t really level at all.”
The JPL/Cal Tech Sea Level Viewer gives some examples of things that can affect the local sea level: El Niño, tsunamis, and hurricanes.
In a global warming context, the sea level has risen an average of nearly two inches since 1993, but it’s a lot higher in some spots than others.
Via What on Earth