NAD27 to NAD83 via NADCON

This transformation represents the integration of the algorithms originally published by the National Geodetic Survey of the US and Geomatics Canada in the forms commonly known as the NADCON program and the National Transformation (Versions 1 and 2). That is, this transformation is the means by which one would convert from the North American Datum of 1927 (NAD27) to the North American Datum of 1983 (NAD83).

All of the techniques encapsulated in this transformation rely on access to data files that define the amount of the shift from NAD27 to NAD83 in a grid form. The coordinate conversion system uses algorithms identical to those used by the respective government-published programs to interrogate the data files and determine the shift for any given coordinate.

Note that the coordinate conversion system considers NAD27 and NAD83 to be a single entity. That is, no distinction is made between Canadian NAD83 or US NAD83 with regard to this technique. In typical operation, the coordinate conversion system will first determine which of the several data files that may be present actually contain coverage for the point to be converted. In the case where one or more files provide coverage, the data file with the densest grid (assumed to be the most accurate) is selected for the calculation. The Geodetic Catalog feature of the coordinate conversion system can be used to specify the data files which are to be considered for use, and a means to control which file takes precedence in the case where one or more of the data files overlap. Refer to Geodetic Data Catalog files for more information.

Note that this transformation implies a conversion from NAD27 to NAD83. The coordinate conversion system will automatically complete the conversion to WGS84 by also causing the "NAD83, No Shift Required" technique after the NADCON/National Transformation technique is done.

The shift data is stored in a single data file for the Canadian National Transformation (either version) and neither of these data files is in the public domain. These data files cannot be provided as part of the normal distribution of this product and must be obtained from Geomatics Canada, and a license fee is often required. In the case of the US NADCON data files, two files are required for each region covered. One file contains the latitude shift and the second contains the longitude shift. These files are in the public domain and are usually included in the distribution of this product. Should updates become available, you can use the data files in the exact form as they are published by the National Geodetic Survey.

Since the coverage of the data files is limited, the coordinate conversion system uses a fallback technique to calculate datum shifts for coordinates that are not covered by the data files. Users indicate the desired fallback technique through the use of the Geodetic Data Catalog file. The Geodetic Data Catalog file named Nad27ToNad83.gdc contains the list of data files to be used for this transformation. Refer to Geodetic Data Catalogs for more information.