Ellipsoid - the mathematical surface which approximates the shape and size of the earth, and is used as a reference frame for position computations. In order to define the geodetic position of a point from the topography, it must be first reduced mathematically onto the ellipsoid and the geoid. Only on these surfaces can a point really have coordinates. Horizontal control surveys determine latitudes and longitudes referenced to a national datum and provide the basis for rectangular coordinate systems. The current primary method for extending horizontal control is precise GPS surveys.
Vertical networks provide elevations of stations above or below mean sea level, the geoid.
Vertical geodetic surveys are adjusted with respect to the geoid. The main method for extending vertical control is still the traditional differential leveling. A geodetic control point may have a precisely determined horizontal position or elevation, but usually not both. This is because the horizontal and vertical positions are developed by different methods and with respect to separate reference surfaces, the ellipsoid and geoid respectively. A Geodetic control network is the wire-frame or the skeleton on which continuous and consistent mapping and surveys are based.