Altitude is an elevation of the earth's surface relative to sea level, measured in meters (or feet). The Earth is not a perfect sphere and it is not a perfect circle, and the elevation of the earth is not a perfectly constant value.

The altitude of land varies by location and depends on the local altitude of sea level. The vertical distance from sea level to the top of a mountain, hill, etc. is called the height of the land. The term elevation is used primarily in the context of the altitude of locations, including regions and countries.

Elevation is a fundamental term in mathematical geodesy and geography, the process of locating a position on Earth. This application is the most commonly used application for sharing the user's altitude or location with their friends.

The barometer shows the pressure difference between the atmosphere and the earth. It can be used to track atmospheric pressure changes.