All heights on the land are calculated from the sea level. Since all seas in the world are connected to each other, more or less the sea level (the top surface) all over the world is taken to be roughly the same. We can easily observe this in the picture given below.
The temple is 100 meters high above sea level. The top of the hill is 150 meters high from sea level. Thus we can say the levels of all the seas are equal in the world.
