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Maps GPS Glossary - D

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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.

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