Your web browser (Safari 7) is out of date. For more security, comfort and the best experience on this site: Update your browser Ignore ALTIMETER altitude meter For the complete encyclopedic entry with media resources, visit: http://www.nationalgeographic.org/encyclopedia/altimeter/ An altimeter is a device that measures altitude, the distance of a point above sea level. Altimeters are important navigation instruments for aircraft and spacecraft pilots who monitor their height above the Earth s surface. Skydivers and mountaineers also use altimeters to pinpoint their location in the sky or on the ground. The most common types of altimeters are barometric. They determine altitude by measuring air pressure. As altitude increases, air pressure decreases. This is because the density of air is lower (thinner) at high altitudes. It exerts less pressure on the Earth below. An altimeter s readings change as elevation changes. The atmospheric pressure on Denali, Alaska, is about half that of Honolulu, Hawai'i. Denali, also known as Mount McKinley, is the highest peak in North America. Honolulu is a city at sea level. Altitude readings can also change due to weather, as air pressure decreases during storms. A simple barometric altimeter includes a sealed metal chamber, a spring, and a pointer that shows altitude in meters or feet. The chamber expands as air pressure decreases and contracts as it increases, bending the spring and moving the pointer. An altimeter can be mounted on an aircraft s instrument panel or worn on a person s wrist. 1 of 5
Other Types of Altimeters Not all altimeters depend on air pressure. The Global Positioning System (GPS), for instance, can provide altitude as part of an area s location by triangulating signals from different satellites. Radar and laser altimeters, found on some aircraft and spacecraft, work similarly to sonar measurements of the seafloor. These altimeters send a radio or laser signal toward the surface and measure the time it takes for the signal to bounce back. The time it takes for the signal to bounce back (or echo) to the aircraft is then translated to an elevation. When used in satellites, radar and laser altimeters are able to combine altitude measurements to create accurate topographic maps of both land and ocean surfaces. The radar altimeter aboard the TOPEX/Poseidon satellite, for example, measured the surface topography of 95 percent of the ice-free ocean. Developed by NASA and CNES, the French space agency, TOPEX/Poseidon s radar altimeter was accurate to within 2 centimeters (less than 1 inch)! Coupled with another satellite, Jason-1, TOPEX/Poseidon graphed the rise in global sea levels, providing evidence of the connection between global climate change and sea level rise. Vocabulary Term accurate adjectiveexact. aircraft vehicle able to travel and operate above the ground. air pressure force pressed on an object by air or atmosphere. altimeter device for measuring altitude. altitude the distance above sea level. 2 of 5
atmospheric pressure barometric force per unit area exerted by the mass of the atmosphere as gravity pulls it to Earth. having to do with atmospheric pressure, or measuring adjective that pressure. chamber sealed compartment. climate change gradual changes in all the interconnected weather elements on our planet. contract verb to shrink or get smaller. decrease verb to lower. density number of things of one kind in a given area. determine verb to decide. device tool or piece of machinery. elevation height above or below sea level. elevation height above or below sea level. evidence data that can be measured, observed, examined, and analyzed to support a conclusion. exert verb to force or pressure. expand verb to grow or get larger. Global Positioning System (GPS) graph verb system of satellites and receiving devices used to determine the location of something on Earth. to depict the relation between certain sets of numbers by plotting them with reference to a set of axes. increase verb to add or become larger. instrument tool. instrument panel laser Term dashboard with an array of dials, lights, and gauges that monitor the performance of a machine or device. (acronym for light amplification by stimulated emission of radiation) an instrument that emits a thin beam of light that does not fade over long distances. 3 of 5
Term location position of a particular point on the surface of the Earth. measurement process of determining length, width, mass (weight), volume, distance or some other quality or size. metal category of elements that are usually solid and shiny at room temperature. monitor verb to observe and record behavior or data. mountaineer someone who climbs mountains. NASA (National Aeronautics and Space Administration) the U.S. space agency, whose mission statement is "To reach for new heights and reveal the unknown so that what we do and learn will benefit all humankind." navigation art and science of determining an object's position, course, and distance traveled. radar (RAdio Detection And Ranging) method of determining the presence and location of an object using radio waves. radio wireless transmission based on electromagnetic waves. satellite object that orbits around something else. Satellites can be natural, like moons, or made by people. seafloor surface layer of the bottom of the ocean. sea level base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle. sea level rise increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year. signal verb to communicate using signs. skydive verb to jump from an airplane in flight and freefall until releasing a parachute. sonar method of determining the presence and location of an object using sound waves (echolocation). spacecraft vehicle designed for travel outside Earth's atmosphere. 4 of 5
Term storm severe weather indicating a disturbed state of the atmosphere resulting from uplifted air. topographic map showing natural and human-made features of the map land, and marked by contour lines showing elevation. translate verb to turn written or spoken text into a different language. triangulation method of determining distance or placement of a point by measuring angles to it from known points. weather state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness. Instructional Content NASA: The Human Altimeter Activity Websites NOAA: Exploring the Ocean Basins with Satellite Altimeter Data NASA: Shuttle Laser Altimeter 1996 2017 National Geographic Society. All rights reserved. 5 of 5