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.