Thanks to some work from the developers working on the Coordinate System functionality, we have some cleaned up directions to replace those in the last Canadian CS post. These steps are both more widely applicable, and easier than the previous directions.
Take a look:
For AutoCAD Map 2012 or later you need to modify the definition of the Geodetic Transformation named "NAD27_to_NAD83". Essentially you will need to edit that definition to include a reference to the newly installed Ntv2_0.gsb file.
- In AutoCAD Map, run the command MAPCSLIBRARY, or press the "Library" button in the ribbon located inside the "Coordinate System" panel.
- locate the Geodetic Tranformation "NAD27_to_NAD83", and select it
- press the "Edit" button
- select the "Grid Data File Interpolation" item in the left pane
- choose the "Grid File Format" "Canadian NTv2" in the combobox
- press the "+" button
- select the Canadian grid file and add it to the list
This dialog box is also where you can specify a different fallback transformation mentioned at the beginning of this readme file.
The NAD27 to NAD83 transformation data files overlap. In the region of overlap, the various files will not produce exactly the same results.
You may indicate which data file is to take precedence by sequencing the file references in the order of precedence. Overlap in this specific case occurs between the NTV2_0.gsb file and the CONUS.L?S files (especially in and around the Detroit area) and between the NTV2_0.gsb file and the ALASKA.L?S files along the common border between Canada and Alaska.
To change the order of the grid files use the up and down arrow buttons on the right side of list of grid files.
- Once your list of grid files is correct, press the save button and close the coordinate system library dialog box
The procedure to use datum shift files provided by a provincial government is similar to the above. You must first obtain the file, copy it into a logical location in the dictionary folder hierarchy, and then make AutoCAD Map aware of the existence and location of the file by editing the appropriate Geodetic Transformation as described above.