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A HARN is a statewide or regional upgrade in accuracy (6-8cm) of NAD 83 coordinates using Global Positioning System (GPS) observations. The datum used with HARN is NAD83(HARN). The HARN grid shift files are used to convert between NAD83 and NAD83(HARN) coordinates. To get the actual datum shift for a coordinate, the 4 closest known coordinates are retrieved from a grid file, and then interpolated using bi-linear interpolation.
For each HARN zone, 2 separate files are needed to calculate the shift. One file containing the latitude shifts (.LAS extension), and a file containing the longitude shifts (.LOS extension). Each file is binary encoded, and contains a header with some information on the file, like rows and columns, bounding rectangle and the grid spacing. The header contents should be the same for both the .los and .las file. Depending on the extension (.las or .los) the file also contains the latitude or longitude datum shifts (in seconds) encoded as 4 byte floating point values. HARN files are provided by the National Geodetic Survey.
HARN grid files can be used in the following areas (sorted by state name):
|California North (above 37 degrees)||cnhpgn|
|California South (below 37 degrees)||cshpgn|
|Idaho and Montana East (east of 113 degrees)||emhpgn|
|Idaho and Montana East (west of 113 degrees)||wmhpgn|
|Puerto Rico and Virgin Islands||pvhpgn|
|Texas East (east of 100 degrees)||ethpgn|
|Texas West (west of 100 degrees)||wthpgn|
When you have to cover larger areas, it is also possible to use NADCON instead. It uses less grid files with a lower grid density. For more information about NADCON click here.
Using the Eye4Software Coordinate Calculator, you can convert coordinates from one map datum to another easily, using the user friendly user interface. This software is shipped with a database containing allmost any map datum, reference ellipsoid and map grid around. The software includes a number of datum and grid definitions that use HARN/NADCON conversion.
For more information, please have a look at the Eye4Software Coordinate Calculator product page.