The lines are arranged to return to the starting point, so that errors can be detected and the data corrected. Types of Surveys Surveying is divided into categories, depending on the area to be surveyed or the purpose of the survey. In plane surveying, the area being measured is small enough that the curvature of the Earth does not need to be considered in the mathematical calculations. A land survey measures the horizontal positions and elevations and is used to establish reference marks to guide construction or to indicate property lines. Engineering surveys are performed to mark out points and lines on the ground for construction projects.
Underground surveys specify locations for drilling tunnels, sewers, or mines or for laying pipe.
A cartographic survey measures the positions of certain features to be included in a map. Cameras mounted on airplanes and sonar devices mounted on ships have added a new dimension to the ancient field of surveying. To conduct an aerial survey, a plane flies back and forth over an area, taking overlapping photographs from which the surveyor can produce a framework. Because the lens of the camera distorts the image of the landscape below, plotting machines and computers are used to create an accurate final image. Before ships and submarines carried echo-sounding devices called sonar, hydrographic surveys, which measure the contour of the bottoms of oceans and lakes, had to be carried out using lead weights attached to long lines.