Maps GPS Glossary - D
datalogger - A handheld data entry computer that is usually used to store data obtained by a GPS receiver.
data message - See navigation message.
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.
DCG - See Depth Controlled Gain.
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.
desired track - Course between the “from” waypoint and the “to” waypoint.
depth controlled gain - Technology used to obtain a much more detailed and accurate picture of the bottom structure using depth, not echo intensity.
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.
dilution of precision - The measurement of geometry of a GPS receiver. DOP indicators: geometric DOP (GDOP), position DOP (PDOP), horizontal DOP (HDOP), vertical DOP (VDOP) and time clock offset (tdOP).
dingle - A small, wooded valley.
distance - The length between your current position and your desired destination. It can be measured in feet, miles, meters, kilometers, etc. .
dithering - The usage of digital noise by the DOD in order to add inaccuracy to GPS signals.
DMA - Defense Mapping Agency - Former name for the National Imagery and Mapping Agency (NIMA).
DOD - The United States Department of Defense that manages and controls the Global Positioning System.
donga - A gully in a veldt.
DOP - See Dilution of Precision.
Doppler-Aiding - The strategy that involves using a measured Doppler Shift in order to help a GPS receiver to smoothly track the GPS signal.
Doppler Shift - The frequency change of a signal caused by the relative motion of the transmitter and receiver.
DOQQ - Digital Orthophoto Quarter-Quadrangle - 1:12,000 scale representations of a quarter of the area of a conventional USGS 7.5 minute topographical map.
downlink - The transmission path sent from a satellite to a ground station.
draw - A small, natural depression or valley in the side of a hill; a gully.
DRG - Digital Raster Graphics - See "Raster Maps."
DTAM - Digital world Tectonic Activity Map.
DTK - See Desired Track.
DTM - Digital Terrain Model - A digital model of the terrain surface - similar to a DEM but often enhanced by planimetric information.
dual-frequency - GPS instruments that can make measurements on both L-Band frequencies.
DXF - Drawing eXchange Format - Used for transfering CAD (Computer Aided Drafting) files.