Surveying, process of measuring distances and features on the earth's surface for map preparation and locating boundary lines. Geodetic surveying, which covers large areas, takes the earth's curvature into account. After a base line of known length is established, the positions of other points are found by triangulation (measuring the angles of the point from each end of the base line) or by trilateration (measuring all the sides of the triangle formed by point and base line). Distances are measured by tape or by electronic means. Instruments for measuring angles are the theodolite and the alidade. Vertical elevations are determined by levels. Much modern surveying is done by photogrammetry, using the stereoscope to determine contours.