NADCON/HARN

Note: These are two different datum shift methods handled under the Nadcon ClassType specified by setting the type parameter.

The NGS NADCON method transforms coordinate values between the North American Datum of 1927 (NAD 27) and the North American Datum of 1983 (NAD 83). The NGS NADCON method provides a transformation that is typically accurate to within 0.15-0.5 meters. The NGS NADCON method applies a simple interpolation algorithm using a gridded set of standard datum shifts as parameters. The shift values for a geographic area are stored in a set of grid files, one that represents latitude shifts (named with the extension .las) and one that represents longitude shifts (named with the extension .los). GeoCalc uses grid files in a format published and provided by the National Geodetic Survey.

GeoCalc supports the definition of a geodetic datum based on a NGS High Accuracy Reference Network (HARN).  The National Geodetic Survey is establishing HARNs within the U.S. on a state-by-state basis.  You can think of a HARN as a geodetic datum, most easily viewed as an enhanced NAD 83 datum. HARNs are also known as NAD 83/91 and High Precision Grid Networks (HPGN). The NGS HARN method is actually very similar to the NGS NADCON method. As with the NADCON method the shift values for a geographic area are stored in a set of grid files, one that represents latitude shifts (named with the extension .las) and one that represents longitude shifts (named with the extension .los). The major difference is that the HARN data files contain shifts from NAD 83 to a HARN instead of NAD 27 to NAD 83. GeoCalc uses grid files in a format published and provided by the National Geodetic Survey.

The "Nadcon" DatumShift has the following Parameters: