All FME features know about their coordinate system and, therefore, their location on the earth. An FME coordinate system contains a complete mathematical model of the conversion between a specific location on the earth and a set of coordinates. Coordinate system definitions are specified by a set of parameters that define this mathematical model, including the earth model (ellipsoid or datum), the units used to measure the coordinates, the projection type, and any parameters specific to the projection type.
Coordinate systems may be extracted from input feature data sources, may come predefined with FME, or may be defined by FME users. FME ships with over 5000 coordinate systems based on a variety of different projections, ellipsoids, and datums. The coordinate systems file coordsys.db in the FME installation folder contains the names and descriptions of all predefined coordinate systems. Some users may wish to use coordinate systems that do not ship with FME, and will need to create their own custom coordinate systems.
FME allows output coordinate systems that are different than the input ones to be specified and performs the required coordinate conversions when necessary. To perform this reprojection, FME uses the CS-MAP reprojection engine, which includes definitions for thousands of coordinate systems, with a large variety of projections, datums, ellipsoids and units.