Why should I be concerned about datums

From Fmepedia

Q) Why should I be concerned about datums?

A) Translations that involve datum shifts, for example NAD27 to NAD83, often require specific grid files which may not be installed with FME. This is particularly true when installing a downloaded version of FME, since to include all grid files would make the download excessively large.

A datum shift without the proper grid file will result in incorrectly projected data.

If you plan on reprojecting data from one datum to another please see the section Obtaining and Installing Grid Files in the Getting Started guide available on the CD or on our website here: www.safe.com/download/index.php#fme_documents

Canadian and US users should be particularly careful to avoid using a grid file for the wrong country. The default behaviour may not be to use your required file. See the online help for more information.