Altitude, the height or elevation of a point or object above a given reference level. In geography and aviation, the reference level is sea level, and altitudes are expressed in feet or meters. In astronomy, the reference level is the horizon, and altitudes are expressed in degrees, as angular distances. In geometry, “altitude” refers to the perpendicular distance from the base of a figure to its highest point. For example, the altitude of a triangle is the distance from its base to the opposite vertex.
Sea level is calculated by averaging heights of the ocean near shorelines. The averages are taken over long periods of time. In the United States, a standard mean (average) sea level has been adopted for the measurement of altitudes. Countries having no shoreline generally use a sea level determined by a neighboring country.
An airplane's altitude is measured with an instrument called an altimeter. An astronomer can determine the altitude of a celestial body by pointing a telescope at it and measuring the angle between the telescope and the horizon. Measurements of angular distance can also be made with an instrument called a sextant.