# Coordinate Systems Analysis: Definitions

Geoid
The equipotential surface in the gravity field of the Earth that coincides with the undisturbed mean sea level extended continuously through the continents. The direction of gravity is perpendicular to the geoid at every point. The geoid is the reference surface for geodetic leveling (surveying) and some inertial navigation systems.
Ellipsoid
A mathematical figure generated by the revolution of an ellipse about one of its axes. The ellipsoid that approximates the geoid is an an ellipse rotated about its minor axis. An ellipsoid serves as the mathematical model from which maps and charts are produced. However, numerous ellipsoids have been developed to support local datums. The use of the WGS 84 ellipsoid provides a single standard of reference within the Department of Defense.
Datum Transformation / Coordinate Conversion
NGA has produced the Mapping Datum Transformation (MADTRAN) software to provide standard transformations between WGS 84 and the major local datums. The NGA developed software Datum Transformation and Coordinate Conversion (DT &CC) is an enhanced version of MADTRAN that allows users to convert coordinates from any two datums.
Datum
A reference surface consisting of the following parameters: the latitude and longitude of an initial point (origin), the orientation of the network, and the two parameters of a reference ellipsoid. Coordinates for a particular ground location will vary based on the datum used to produce a particular map or chart. Therefore, it is essential that the datum used to derive the coordinates be included when reporting positions. WGS 84 now provides the single standard reference datum, or gegraphic reference system within the Department of Defense.
Map Projection
An orderly system of lines on a plane surface representing a corresponding system of parallels of latitude and meridians of longtitude of the Earth or section of the Earth.
Graticule
A network of lines representing parallels of latitude and meridians of longitude.
Grid
Two sets of parallel lines intersecting at right angles and forming squares. A grid is superimposed on maps, charts, and other similiar representations of the Earth's surface in an accurate and consistent manner to permit identification of ground locations with respect to other locations and the computation of direction and distance to other points.
Reference Systems
Any method of position referencing and reporting (coordinate system) is dependent upon the ellipsoid and datum used to model the Earth. Any distortions or inaccuracies in the sources of the coordinate, whether from topograhpic map, aeronautical or hydrographic chart, digital data product, or other source can be compounded if different coordinates based on different datums are mixed when reporting positional information. For this reason, it is important to state the reference datum when using any of the grid or geographic systems defined above.

Point of Contact: Coordinate Systems Analysis Team
phone (314) 676-9124, DSN 846-9124
coordsys@nga.mil