A custom coordinate system is a Shared Resource that defines one or more coordinate systems that are not included in FME by default.
FME is used in many different countries around the world, and it is also used with a variety of custom datasets within each country. However, sometimes FME’s Coordinate System Gallery doesn't include the coordinate systems that are being used. In these cases, you will have to define the coordinate systems before FME can use them.
coordsys.db in the FME installation directory contains the names and descriptions of all predefined coordinate systems. It is typically a subset of all the definitions that are known to FME (i.e. there may be some additional systems defined in the coordsys file that aren't in the coordsys.db either because the systems have been deprecated, or some other reason). Any systems defined in LocalCoordSysDefs.fme, MyCoordSysDefs.fme or as an FME Shared Resource are automatically added to FME and don't need an entry in this file.
To allow sites to add their own coordinate systems, a file of local coordinate system definitions is automatically loaded and made available to each FME session. This file is called LocalCoordSysDefs.fme, and resides in the Reproject subdirectory under the FME installation directory. It contains a series of COORDINATE_SYSTEM_DEF, DATUM_DEF, ELLIPSOID_DEF, and UNIT_DEF lines that define additional, site-specific coordinate systems. You can edit these files to add your own definitions.
LocalCoordSysDefs.fme is a file that contains overrides to the standard coordinate systems (and related parameters) defined by FME. The built-in definitions for any coordinate system/datum/ellipsoid may be overridden if by a text defintion in the Reproject/LocalCoordSysDefs.fme file. FME ships with a number of overrides (and supplemental) systems in this file. The structure of LocalCoordSysDefs.fme is the same as the custom coordinate system file MyCoordSysDefs.fme, and its contents can be used as a template for new custom coordinate systems. NB: It is recommended that users should NOT edit LocalCoordSysDefs.fme, because each time FME is installed, it will overwrite this file with whatever is current for that FME.
MyCoordSysDefs.fme in the Reproject directory contains custom coordinate system definitions.
WARNING:
Although FME does not overwrite MyCoordSysDefs.fme during installation, it is always good practice to back up the file anyway. In addition, although you should not have to edit either the LocalCoordSysDefs.fme and coordsys.db files, FME overwrites these files during installation. Therefore, you will have to back up the files if you have made any changes.