NAD27-83 Coordinate Conversion

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Redefinition of the North American reference datum involves the transformation of geographical coordinates from the former North American Datum 1927 (NAD27) (Clarke 1866 reference ellipsoid) to the North American Datum 1983 (NAD83) on the Geodetic Reference System 1980 (GRS80) or the equivalent World Geodetic System 1984 (WGS84).

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To update or merge NAD27 and NAD83 geocoded image and map data would require the rectification of old images to new NTS map sheet standards which are in NAD83. This could be done through the reprocessing of raw image data with ground control points whose UTM coordinates were projected into the NAD83 framework. However many users have applied specific processing to their data and will not want to start at the raw level a second time.

The relationship between NAD27 and NAD83 includes the change in the reference ellipsoid as well as modeling of the NAD27 distortions which exhibit regional trends in certain areas and regions where the distortions appear random. Given the variability and complexity of the distortions, it is of practical importance to simplify the transformation, thereby rendering both the distortion and ellipsoid changes to within acceptable tolerences. Existing geocoded images may then be transformed and knowledge of the incurred positional error taken into account.

The colour-coded map shows the maximal absolute error of a rigid-body shift in the image when the components of shift, both Northing and Easting, were determined from a single point located at the center of the map sheet. The colour values are tiled on the 1:50 000 NTS map sheet coverage for Canada and are based on commonly used pixel dimensions. This value is the maximum difference in shift between a one-point transformation and the exact shift for each of the four image corners. The direction of the error is not considered and may therefore be either in Northing or Easting.

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Remote sensing users wishing to compute their own transformation parameters can use on-line facilities at http://www.geod.nrcan.gc.ca. Specifically, use the program GSRUG to convert between geographic and projected coordinates, and the NTv2 system to calculate transformations to/from NAD27 and NAD83.