NADCON Extension Documentation

The NADCON extension is an Avenue implementation of NOAA's NADCON program; it transforms lat/long shapefiles from NAD27 to NAD83 and vice versa. (Click HERE for important information on ArcView and datums.)

NADCON.ZIP contains the following files:

NADCON.AVXThe extension
CONUS.DBFGrid table for conterminous United States
CONUS.PRMParameter file for grid table

When the extension is installed, a "NADCON" menu item appears under the "Theme" menu in a View.

Active theme(s) in a view will be converted. The coordinates of the underlying data must be decimal degrees. When executed, NADCON asks the user for:

  1. The grid file to use (see below on using other grid files)
  2. The conversion option ("NAD27 to NAD83" or "NAD83 to NAD27")
  3. Add transformed shapefiles to a View
  4. For each active theme, the output shapefile
WARNING: Large shapefiles (> 1Mb) may take a very long time to process, and thus should be handled one at a time.

For more information, visit:

Or view the documentation at the download site:

Can I use other grid data?

Other grid files, such as Alaska or Hawaii, may be imported and used by the extension. The program NADGRD is required. For example, to process Hawaii:

  1. Download HAWAII.LAS, HAWAII.LOS, and NADGRD.EXE from the above FTP site to a temporary directory.
  2. At an MS-DOS prompt, run NADGRD.
  3. Hit ENTER to choose binary input file format and enter "HAWAII" as the input filename.
  4. Choose "A" as the output file format and hit ENTER to use the default output filename.
  5. Hit ENTER to use the default values for minimum latitude, maximum latitude, minimum longitude, and maximum longitude.
  6. In ArcView, load the NADCON extension, open a Script Editor window, and enter the following code:

    result = av.Run("NAD.GrdImport",{})
    Compile and run it to create HAWAII.DBF and HAWAII.PRM from HAWAII.LOA and HAWAII.LAA.

Can I improve the performance?

Performance of the NADCON extension is not improved by reducing the size of the grid file to cover a single state. It might be possible to package the grid data into a DLL, but I'll leave that as an exercise for someone else!

What if my shapefiles aren't lat/long?

The Projector! extension, included in the ArcView samples, may be used to convert shapefiles to lat/long. When unprojecting, ArcView assumes the same datum as the projection. Thus the following script:
' UTM Zone 12, NAD27

p1=Point.Make(409023.093750,3901027.500000) ' -112, 35.25
p2=Point.Make(409303.250000,3928754.000000) ' -112, 35.5
p3=Point.Make(431977.937500,3928553.000000) ' -111.75, 35.5
p4=Point.Make(431767.812500,3900827.250000) ' -111.75, 35.25

theRect = Rect.MakeXY(-112,35.25,-111.75,35.5)
thePrj = TrnMerc.Make(theRect)
theSpheroid = thePrj.GetSpheroid

up1 = p1.ReturnUnprojected(thePrj)
up2 = p2.ReturnUnprojected(thePrj)
up3 = p3.ReturnUnprojected(thePrj)
up4 = p4.ReturnUnprojected(thePrj)
will return the corner points of theRect.