Editing Geophysics (section)

===Geodesy===
{{Main|Geodesy}}

Geophysical measurements are generally at a particular time and place. Accurate measurements of position, along with earth deformation and gravity, are the province of [[geodesy]]. While geodesy and geophysics are separate fields, the two are so closely connected that many scientific organizations such as the [[American Geophysical Union]], the [[Canadian Geophysical Union]] and the [[International Union of Geodesy and Geophysics]] encompass both.<ref name=NRC>{{harvnb|National Research Council (U.S.). Committee on Geodesy|1985}}</ref>

Absolute positions are most frequently determined using the [[global positioning system]] (GPS). A three-dimensional position is calculated using messages from four or more visible satellites and referred to the [[GRS 80|1980 Geodetic Reference System]]. An alternative, [[astro-geodetic|optical astronomy]], combines astronomical coordinates and the local gravity vector to get geodetic coordinates. This method only provides the position in two coordinates and is more difficult to use than GPS. However, it is useful for measuring motions of the Earth such as [[nutation]] and [[Chandler wobble]]. Relative positions of two or more points can be determined using [[very-long-baseline interferometry]].<ref name=NRC/><ref>{{harvnb|Defense Mapping Agency|1984}}</ref><ref name=Torge>{{harvnb|Torge|2001}}</ref>

Gravity measurements became part of geodesy because they were needed to related measurements at the surface of the Earth to the reference coordinate system. Gravity measurements on land can be made using [[gravimeters]] deployed either on the surface or in helicopter flyovers. Since the 1960s, the Earth's gravity field has been measured by analyzing the motion of satellites. Sea level can also be measured by satellites using [[Radar altimeter|radar altimetry]], contributing to a more accurate [[geoid]].<ref name=NRC/> In 2002, [[NASA]] launched the [[Gravity Recovery and Climate Experiment]] (GRACE), wherein two twin [[satellite]]s map variations in Earth's gravity field by making measurements of the distance between the two satellites using GPS and a microwave ranging system.  Gravity variations detected by GRACE include those caused by changes in ocean currents; runoff and ground water depletion; melting ice sheets and glaciers.<ref>{{harvnb|CSR|2011}}</ref>