When features are reprojected (using the CS-MAP reprojection engine) between Swedish coordinate systems based on SWEREF99 and RT90 datums, a method known as "direct projection" is used. Direct projection replaces datum shifts with custom projections that closely approximate the datum shifts.

FME's algorithm for Swedish direct projection is as follows.

RT90 to SWEREF99 systems:

1. Reproject the feature to the RT90 2.5 gon V variant with a datum that matches the source coordinate system datum.
2. Tag the feature with the RT90 2.5 gon V variant on the SWEREF99 datum.
3. Reproject the feature to the destination system.

SWEREF99 to RT90 systems:

1. Reproject the feature to the RT90 2.5 gon V variant on the SWEREF99 datum.
2. Tag the feature with the RT90 2.5 gon V variant with a datum that matches the destination coordinate system datum.
3. Reproject the feature to the destination system.

Coordinate systems on the RT90, RT90-3-7P, and SWEREF99 datums are eligible for direct projection.

Swedish direct projection has been used in FME for many years for classic geometry and attribute reprojection. As of FME 2012 beta build 12003 it applies to rich geometry as well.

The CS-MAP reprojection engine is no longer the preferred method for performing Swedish reprojections. Instead, we recommend the GTRANS reprojection engine developed by Lantmäteriet. FME includes a GtransReprojector transformer to use instead of the Reprojector.

For more information see Lantmäteriet’s reference on Swedish direct projection.