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NAD 27 - North American Datum 1927

NAD 83 - North American Datum 1983

nadir - A point on the celestial sphere directly below the observer, diametrically opposite the zenith.

nala - A ravine.

National Differential Global Positioning System - A network of ground- and beacon-based differential stations that increases GPS accuracy.

National Map Accuracy Standards - (NMAS ) � Specification given by the U.S. Office of Management and Budget that define accuracy standards for published maps.

National Marine Electronics Association - A US standards committee that defines the data message structure, contents, and protocols that allow GPS receivers to communicate with other electronic equipment.

nautical mile - a unit of length used in sea and air navigation, based on the length of one minute of arc of a great circle - equal to appr. 1852 meters (6076 feet).

NAVD 88 - Vertical reference system (elevation) for U.S., Canada, and Mexico; orthometric heights are derived from a particular point on the St. Lawrence Seaway.

navigation - The process of determining the direction or heading of a plane, ship, automobile or person.

navigation message - A message sent by a GPS satellite that contains satellite's status and an indication of its health, ephemeris data, their locations and PRN numbers.

NAVSTAR (Navigation Satellite Timing and Ranging) - The name given to the system of GPS satellites.

NDGPS - Nationwide DGPS.

neatline - Line separating the body of a map from the map margin. On a standard quadrangle map, the neatlines are the meridians and parallels delimiting the quadrangle.

NGA - National Geospatial-Intelligence Agency � A US agency that is primary purpose is to collect, analyze and distribute geospatial intelligence (GEOINT) in support of national security.

NGS - National Geodetic Survey - An agency of the United States NOAA that defines and manages a national coordinate system. This network, the National Spatial Reference System (NSRS), provides the foundation for transportation and communication; mapping and charting; and a multitude of scientific and engineering applications.

NIMA - National Imagery and Mapping Agency � The former name of NGA (National Geospatial-Intelligence Agency).

NMEA - see National Marine Electronics Association

north up display - This setting allows a GPS receiver user to set it so that north will always be located at the top of the screen.

North Pole - The surface located in the northern hemisphere where the Earth's axis of rotation meets its surface. It is also known as the Arctic.

NSDI - National Spatial Data Infrastructure - The set of base geographic data necessary for effective operation of the United States federal government and its suppliers, made accessible as a distributed database.

NSRS - National Spatial Reference System - A system administered by the NGS that provides the foundation for transportation and communication; mapping and charting; and a multitude of scientific and engineering applications.

nullah - See nala.

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