Problem: Latitude values are incorrect when using NADCON in ArcMap


Two data sets may not line up when transforming from NAD 1927 to NAD 1983 in the United States. This is not a problem when converting from NAD 1983 to NAD 1927.


The NADCON grid is automatically loaded into memory when ArcMap is started. If the grid is not loaded into memory, the results differ by less than a meter.

Solution or Workaround

This issue has been addressed in ArcGIS Version 8.1.2. If you are using a previous version, reset the NADCON transformation.

  1. Right-click the Data frame > Properties.
  2. Switch to the Coordinate System tab.
  3. Click Transformations.

    Make certain NAD 1927 is listed in the 'Convert from' box and NAD 1983 is displayed in the Into dropdown list.

  4. In the Using dropdown list, select another transformation, such as GT_NAD_1927_To_NAD_1983_Alaska.
  5. Click OK.
  6. Click OK on the Data Frame Properties dialog box.