Elevation is a measure of height above or below a fixed reference point, most commonly a reference geoid, which is a mathematical model of the Earths sea level as an equipotential gravitational surface. The term "elevation" is mainly used when referring to points on the Earths surface, while "altitude" or "geopotential height" is used for points above the surface, such as an aircraft in flight or a spacecraft in orbit, and "depth" is used for points below the surface. Elevation is usually measured in meters or feet and can be shown on maps by contour lines that connect points with the same elevation. Maps that show elevations are called topographic maps. Elevation plays a crucial part in everyday life, as it is used in architecture and engineering to determine the center of gravity and the type of materials to be used, and it can help predict and prevent disasters such as flooding.