Geodesy, << jee OD uh see, >> is a science that seeks to determine the size, shape, and gravitational field of the earth. It also monitors variations over time in the earth’s rotation, size, and topography. Geodesists use various surveying techniques to measure the distances and directions between points on, under, or above the earth’s surface. They calculate the elevation, latitude, and longitude of thousands of places on earth. One major use of geodetic data is the making of a wide variety of maps, from navigational charts to topographical maps.
Since the mid-1960’s, space technology has greatly improved the accuracy of geodetic measurements. Geodesists are making measurements with radio waves emitted by quasars–extremely luminous objects in distant galaxies–and with laser beams reflected off artificial satellites. Using these tools, geodesists are building a global reference frame (a listing of measurements of the locations of various points) that is accurate to 1 centimeter (3/8 inch).
Geodesists also use these tools to directly measure the movements of the plates that form the earth’s surface with an accuracy of a few millimeters a year. Geodesists map movements of the earth’s crust associated with earthquakes and subsidence, the sinking of the crust caused by withdrawal of fluid under the surface. In addition, they track variations in the earth’s rotation and in the wobble about its axis.
