NADCON is a datum conversion method developed by the NGS (National Geodetic Survey), which is used in the United States to convert coordinates between the North American Datum of 1927 (NAD27) and the North American Datum of 1983 (NAD83) with high precision (12-18 cm).
NADCON can also be used in Puerto Rico, American Samoa and the Virgin Islands. 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 NADCON 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. NADCON files are provided by the National Geodetic Survey.
NADCON grid files can be used in the following areas:
|Alaska and Aleutian Islands||alaska||46N||77N||194W||128W|
|St. Lawrence Island||stlrnc||62N||64N||172W||168W|
|St. George Island||stgeorge||56N||57N||171W||169W|
|St. Paul Island||stpaul||57N||58N||171W||169W|
|Puerto Rico and the Virgin Islands||prvi||17N||19N||68W||64W|
For higher precision, you can use the High Accuracy Reference Network (HARN), also called High Precision Geodetic Network (HPGN).
It uses more grid files with a higher grid density, allowing higher accuracy (6-8cm).