geodesy (jēŏdˈĭsē) [key] or geodetic surveying, theory and practice of determining the position of points on the earth's surface and the dimensions of areas so large that the curvature of the earth must be taken into account. It is distinguished from plane surveying, the operations of which are executed without regard to the earth's curvature. In geodetic surveying, two points, called stations, many miles apart are selected, and the latitude and longitude of each is determined by astronomical means. The line between these two points, the base line, is measured with a high degree of accuracy. The position of a third station is determined by the angle it makes with each end of the base line. This process, called triangulation, is continued until the whole area to be surveyed is mapped. For indicating a triangulation station the U.S. Coast and Geodetic Survey uses a bronze disk suitably marked and having a projection on the bottom for anchoring it in concrete. Where the curvature of the earth is great or where there are hills or high trees between stations, towers are built so that one station may be seen from another. In recent years, artificial earth satellites have come into wide use as geodetic instruments. Shifts in the orbits of the satellites Explorer I and Vanguard I provided data by which geodesists corrected the value for the oblateness of the earth. This led to a program of geodetic satellites specifically designed to measure variations in the earth's gravitational field, and to determine the exact geographic position of points on the earth's surface. A triangulation station in space, the geodetic satellite, is photographed against the background of stars in order to compare accurately the relative positions of points on the earth.