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.
When an FME reader knows its coordinate system, the FME creates a coordinate system to match the input data specification and tags each feature read with this system name. However, a majority of formats do not explicitly store the coordinate system. In such cases, mapping file directives are used to supply coordinate system information.
The mapping file may also contain a specification as to which coordinate system the output data is to be in. If the output coordinate system is specified and it is different from the input coordinate system, then the FME automatically converts the feature data between coordinate systems. For example, converting features from one coordinate system based on North American Datum (NAD) 27 to another from NAD83 causes the FME to perform the required datum conversion. To ensure that the reprojection is accurate, FME automatically vectorizes arcs, ellipses, rectangles, and rounded rectangle objects as necessary before performing the coordinate system change. The output system, or format, stores the coordinate system of the features if it has facilities for doing so; for example, the MapInfo formats store the coordinate system of the data.