Canadian National Transformation

Q:  How can I use the Canadian National Transformation for NAD27-NAD83 coordinate system conversions?

A:  Geomatics Canada (the agency for producing and providing geographic information for Canada) defines a coordinate conversion function that is different from the “NADCON” function used by the United States Geological Survey.  If you have a copy of the Geomatics conversion data file you can modify a configuration file to use that function for the TAP datum conversions between NAD27 and NAD83 in Canada for the Canadian Spatial Reference System.

A stand-alone conversion utility (actually a DOS function) is available from Geomatics Canada.    The same data file used for the coordinate conversion by this utility can be used to implement the conversion in TAP.

The file is named NTV2_0.GSB and is provided with the utility downloaded from Geomatics Canada.  Place a copy of the NTV2_0.GSB file into the “CoordSys Data” folder in the directory where TAP is installed.  There will be numerous other files in this folder.  No changes to these other files should be made except as described below.  Changing these files will cause unpredictable operation of your TAP system.

In the same “CoordSys Data” folder, find the file named Nad27ToNad83.gdc and make a backup copy of this file in case any problems are encountered in the simple edit process.

Edit the Nad27ToNad83.gdc file with a text-only editor.

Find the following line in the file:

#.\Ntv2_0.gsb        #version 2 of Canadian National Transformation

On that line, remove the initial “#” comment character, so the line is changed as shown:

.\Ntv2_0.gsb        #version 2 of Canadian National Transformation

Save the Nad27ToNad83.gdc file. 

With those changes made, datum conversions between NAD27 and NAD83 in Canada will use the Geomatics function instead of the USGS function.

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