HARN to NAD83 via NADCON

HARN (High-Accuracy Reference Network) has also been known as HPGN (High-Precision GPS Network): both terms refer to NAD83/91, which is a rework of NAD83 with the aid of GPS technology (since GPS was not functional in 1983). This technique selection implies the use of the algorithms and data files of the U. S. National Geodetic Survey's NADCON program to effect the shift between NAD83 and HARN.

As with the NADCON technique, this transformation relies on the existence of data files that define the shift at various geographic points in a grid format. As is the case with the NAD27/NAD83 NADCON data files, these data files come in pairs and are in the public domain. A Geodetic Data Catalog file is used to inform the coordinate conversion system of which files are to be considered for use, and the location of these data files.

All of the data files used in this transformation adhere to a specific naming convention (as published by the National Geodetic Survey): they must have the proper .LAS and .LOS extensions, and the names and locations must be properly recorded in the Geodetic Data Catalog file. These files all overlap their neighbors by a substantial amount.

Since different results for the same point can be obtained depending upon which specific data files are used, users should pay significant attention to the setup of the Geodetic Data Catalog for specific data conversions. For example, if the geography one is working is primarily in Ohio, then the Ohio HPGN file should be listed first in the catalog file. This will cause that data file to take precedence over all others in the case of overlap.

Users can use the Geodetic Data Catalog file HarnToNad83.gdc to specify a fallback definition to be used when coordinate data not covered by the data files is processed. Refer to this file for more information. Since the shift produced by this transformation is relatively small (e.g., 1 or 2 feet), the fallback feature is rarely used.