MicroStation can reproject design data from one geographic coordinate system to another. When a new GCS is selected and the reproject option is chosen in the confirmation dialog, the data within the model to which you are assigning the GCS is permanently reprojected. When a reference attached with the geographic reprojection option is loaded, the data within the referenced model is reprojected temporarily and not written back to the file.

Geographic reprojection involves these steps for each point that must be reprojected from the source GCS to the destination GCS. If the source or destination GCS is non-projected, all these steps are not necessary since the Cartesian coordinates in those situations are actually longitude and latitude rather than actual linear values.

1. If necessary, model coordinates are converted from the model’s linear units to Cartesian coordinates in linear units of the source GCS.
2. Cartesian coordinates are converted to longitude, latitude, and elevation in the source GCS datum.
3. If the datum of the source GCS is different from the datum of the destination GCS, the datum shift calculation is performed to give the longitude, latitude, and elevation in the datum of the destination GCS.
4. The longitude, latitude, and elevation are converted to the Cartesian coordinates in the linear units of the destination GCS.
5. If necessary, the Cartesian coordinates in the units of the destination GCS are converted to the model’s linear units.

The reprojection algorithm is performed for each point of elements that contain discrete points such as line strings, lines, B-splines, and point strings. For other element types, the reprojection technique varies and can be influenced by the reprojection settings that are in effect.

There are separate reprojection settings for reference reprojection and active model reprojection. Since reference reprojection happens every time a reference is loaded, it makes sense to select faster, more approximate settings than those that are used when the active model is reprojected when you select a different GCS.