Altimeters measure the device's altitude above ground or sea level to provide the user with navigation information. Many altimeters use atmospheric pressure to determine altitude by calibrating a barometer with a nonlinear relationship between local pressure and temperature. Radar signals can be employed to ascertain an object's height above ground level by measuring the time taken for the signal to travel between the object and the surface. Radar altimeters are used in terrain awareness and warning systems to avoid collision with the ground. Global Positioning System receivers can be utilized to determine an object's altitude through trilateration. Altimeters are important flight instruments used in aircraft. Hand-held and wrist-mounted altimeters are used in hiking and climbing applications to verify location when used in combination with topographic maps. Satellite-based altimeters measure ground and sea surface height to provide maps of surface topography.
Images credits: Wikimedia; Trendy Gadget