Surveying is the science of determining the three-dimensional position of points, distances, and angles. Most of the time, surveyors are employed by the government to make maps, establish national boundaries, and demarcate private property lines. They also assist attorneys in drafting legal documents pertaining to land usage. There are many different types of local surveyors that use global positioning systems. Geodetic surveyors measure large areas of land, usually for building contractors. Geophysical prospective surveyors map places below the earth's surface. They are employed by oil companies for obvious reasons. Marine or hydrographic surveyors concentrate on measuring bodies of water including rivers, lakes, harbors, and the ocean. Geographic information systems specialists create the maps for technology companies using satellite imagery. Local engineers and land surveyors can't rely on data obtained from consumer electronics. They need to be able to access information about the land below the surface of the earth or miles above ground. GPS imaging has increased the speed of surveying, but these systems are not always accurate, or at least, not accurate enough for professional surveyors. There are also certain types of terrain where GPS equipment does not work very well. For example, a surveyor would not be able to use GPS technology in a tropical rain forest because these devices do not work in places that have too many trees. High precious GPS equipment is available for these professionals, but at a most high cost than models designed for commercial sale. There are also software programs that utilize GPS technology.