Grid Shift

From Fmepedia

A grid shift (also known as a Datum Shift) occurs when data is reprojected between coordinate systems that have different datums, and needs to be offset to appear in the correct geographical location. In theory a datum shift is the generic term for the process (which could be carried out by mathematical calculation of the offset), whereas a grid shift is specifically the offset of data using a file containing a grid of pre-defined offsets.

FME employs the grid shift method. When reprojecting data between different datums ALWAYS check the log file to make sure the correct grid shift file is being used: it is difficult to notice this problem by simply looking at the data.