Q&A: What are grid shift files and how are they managed with FME?

Question

What are grid shift files and how are they managed with FME?

Answer

Grid shift files are used when reprojecting between coordinate systems that have different datums.

FME supports conversions between coordinate systems using different datums. Many datum transformations are not mathematically definable and require the use of grid shift files. If you attempt to make a datum transformation of this kind without the appropriate grid shift file in place FME will stop the translation.

The FME installation includes a configuration file for managing the use and location of grid shift files. These files are found in the Reproject directory in the FME installation directory. For example the file Nad27ToNad83.gdc includes file paths to both the Canadian and US grid shift files for the NAD27 <-> NAD83 transformations.

These configuration files can be edited using Tools > Options > Coordinate Systems from the FME Workbench menubar. See the FME user documentation on how to add a new grid shift file.

Please note that different jurisdictions and standards may require the use of specific grid shift files not necessarily provided by Safe Software. In these cases, you will need to acquire the appropriate grid shift files and edit the relevant .gdc file in the FME Reproject directory to make FME aware of these files.

Please download reprojection grid files from the FME Downloads page, under Reprojection Grid Files.