Maps GPS Glossary - DDDD datum - (pl. datums) A model of the earth's surface that is comprised of a network of physical points that serves as a reference for maps, for both horizontal and vertical data. In the U.S. early topographic maps were based on the North American Datum 1927 (NAD27). More recent maps are based on a North American Datum 1983 (NAD83). GPS uses a datum called World Geodetic System 1984 (WGS84). There are over 100 datums in use around the world. dead reckoning - The technique of determining position by computing distance traveled on a given course. Distance traveled is determined by multiplying speed by elapsed time. degree - A unit of measurement; 360 degrees equal one complete circle, as around the earth on a line of latitude; measured from 0 to 90 degrees from the equator to each pole along a line of longitude. depression - An area in the surface of the earth that is lower in elevation than the surrounding area; a hole. depression contour - A contour line with hachures (small perpendicular tick marks) to signify a depression. DGN - microstation design file - A file format/extension. DGPS - Differential GPS - A method of improving GPS accuracy using measurements at a known location to improve the measurements made by other GPS receivers within the same general area. dingle - A small, wooded valley. DMA - Defense Mapping Agency - Former name for the National Imagery and Mapping Agency (NIMA). donga - A gully in a veldt. |