2. The branch of science that deals with the precise measurement of the size and shape of the earth, the mapping of points on its surface, and the study of its gravitational field.
3. A subdivision of geophysics which includes the determination of the size and shape of the earth, the earth's gravitational field, and the location of points fixed to the earth's crust in an earth-referred coordinate system.
4. The determination of the geometry of the earth's surface (both solid and liquid), including the time variability of this geometry.
Determination of the earth's orientation is essential because a number of measurements of geometric quantities involve observations of extraterrestrial objects (artificial satellites, radio stars, etc.). These measurements can not be interpreted without knowledge of the earth's orientation.