An altimeter is a tool that tells you how high you are. You meet it first in a plane: pilots watch it so they do not fly too low or too high. Some hikers' watches and climbing gear have a small one too. It does not tell you how fast you are going, and it does not tell you where you are on a map — only height. Though actually, the number it shows is often height above the sea, not “how far from the ground under you right now.” Over mountains those two ideas come apart, which is why pilots still need their other instruments as well.

高度計