This article was contributed by Esri Canada.
A transformation is required when projecting a dataset between two different datums, whether on-the-fly in ArcMap or when using any of the Project tools from ArcToolbox. Equation-based and grid-based transformation methods are both supported in ArcGIS. Equation-based transformations use either a 3-parameter shift (dX, dY, dZ) or a 7-parameter shift (dX, dY, dZ, rX, rY, rZ, ds) to translate between coordinates. Grid-based transformations use binary files that contain the differences between the two geographic coordinate systems to perform a bilinear interpolation.
The supported transformation methods for ArcGIS 9.x and 10.x are described in the article: How To: Select the correct geographic (datum) transformation when projecting between datums.
In ArcMap, the transformation method can be selected when projecting on-the-fly, through the Data Frame Properties (
), and when using the Project tool (
Transformations for Canada
The National Transformation version 2 (NTv2) is a grid-based transformation method originally created by the Geodetic Survey Division of Geomatics Canada to transform between NAD 1927 and NAD 1983 across Canada. The grid shift file (.gsb) can be downloaded from the COSINE Online Service website, for more information refer to the article on how to Download the National Transformation version 2 files for Canada.
Many Canadian provinces, in conjunction with the NTv2 national standard, have created grid-based transformations files (.gsb) to transform between the various provincial datums. Some of these Canadian transformation methods are recognized by the EPSG Geodetic Parameters database. However, other provinces have not completed the internationalization process, so their grid shift files are not yet listed as supported transformations. To learn more about working with these files as custom geographic transformations, refer to the web help topic: Create Custom Geographic Transformation.
The Grid-based Transformation Methods for Canada PDF is a list of grid-based transformation methods recognized by the EPSG Geodetic Parameter database. To use any of these Canadian grid files, copy them into one of the following directories:
• For ArcGIS 9.x - C:\Program Files\ArcGIS\pedata\ntv2\canada
• For ArcGIS 10.x - C:\Program Files\ArcGIS\Desktop10.0\pedata\ntv2\canada
If the Canada folder does not exist, create it.
WARNING: Prior to ArcGIS 10.0, there is a known issue that the Project Tool executes even if the grid shift file is not on disk and the output is not correct. The .gsb file must be present in one of the directories specified above!