To locate and place any object in the plane or in the space, to define their location are enabled by coordinate systems. In the coordinate systems, or, in other words, the reference systems, the coordinates of the objects describe its location exactly. The axes to the coordinate systems are linearly independent from each other. The system types in the GIS practice:
planar orthographic coordinate system (planar system)
spatial orthographic coordinate system (or Cartesian system, after the Latin name of Descartes)
spherical polar coordinate system (geocentric or spherical system)
ellipsoidal (geodetic) coordinate system
The axes of the first two types are lines, perpendicular to each other in the plane or in the space, respectively. In the last two cases, the coordinates are one distance (from the center, or more practically, from a defined surface) and two directional angles, the longitude and the latitude. The coordinates are given in units described in Point 2.1.
Neither the coordinate systems nor the coordinates themselves are visible in the real world. That’s why the coordinate systems are realized by physically discrete points and their fixed coordinates in a specific system. This physically existing, observable point set, characterized by point coordinates is called reference frame. In fact, all geodetic point networks are reference frames. Any reference frame is burdened by necessary errors, by theoretical or measure ones, based on the technology of the creation of the frame. In case of the geodetic frames, the difference between the Earth’s theoretical shape, the geoid, and its ellipsoidal approximation causes theoretical errors. Besides, the limited measuring accuracy results further errors in the coordinate frame.
Longitude of a point is the same both in spherical (geocentric) and ellipsoidal (geodetic) systems. However, its latitude is different, because of the altered definition of the angle of the latitude. In this version of the textbook, all latitudes and longitudes are interpreted in ellipsoidal (geodetic) system.