Is the height of a mountain like Mt Everest always measured from sea level?

Answer 1

The height of a mountain is normally calculated from mean sea level. The height of a mountain above its base is known as its topographical prominence, and the recognized height of Mount Everest was based on this height rather than its height above mean sea level until recently.

Answer 2

It depends on which area you take flight from. A 5000-mile radius, from the height of the plane all the way down to ground level, is implied in all areas around the ground level of where the plane is. The landscape surrounding the 5000-mile radius will of course be uneven, but an onboard averaging instrument accurately measures out the altitude of the landscape itself. From there, the instrument evenly averages the altitude. That is how altitude is measured these days.