The Geodetic Glossary

The horizontal angle measured clockwise from a north base line or meridian. In surveying, it is commonly used to define the direction of a line relative to True North or Grid North.

A reference system or an approximation of the Earth's surface against which positional measurements are made. A geodetic datum defines the size and shape of the earth and the origin and orientation of the coordinate systems used to map the earth.

A mathematical surface formed by revolving an ellipse about its minor axis. It is used as a smooth approximation of the Earth's physical surface for coordinate calculations. Examples include WGS84 and GRS80.

The equipotential surface of the Earth's gravity field which would coincide with the ocean surface if it were at rest and could be extended through the continents. It represents the "true" physical shape of the Earth, unlike the smooth ellipsoid.

The Military Grid Reference System (MGRS) is the geocoordinate standard used by NATO militaries for locating points on the earth. It is derived from the Universal Transverse Mercator (UTM) grid system.

Universal Transverse Mercator. A system of map projection that uses a 2-dimensional Cartesian coordinate system to give locations on the surface of the Earth. It divides the world into 60 zones.