NAD27 NAD83 Datum Shifts In US and Canada

From fmepedia

FME supports two algorithms, each with their own grid files, for the NAD27/83 datum shift.

NADCON is used in the United states, and NTv2 is used in Canada. It is important to configure FME to use the correct method for your data. In Workbench, you can access this setting under Tools -> Options -> Coordinate Systems -> NAD27<->NAD83. We recommend that you ensure the grid files you need are in the list, and the ones you do not need are removed.

The following figure shows the difference between the NADCON and NTv2 NAD27/83 grid shifts in the US and Canada:

The source data is attached as Nadcon_ntv2_difference.zip below. There are two shapefiles inside. "Contour" contains 1m contours of the differences, and "Overlap" contains the polygon boundary of the overlapping area.