Project from WGS to NAD

The most accurate way to convert between WGS84 or WGS72 and NAD27 is the following three steps: from WGD84 or WGS72 to nad83 and then to nad27.

1. PROJECT the dataset from "DATUM WGS72" to "DATUM NAR_D" (uses three parameter method). eg. project file creates a cover in "DATUM NAR_D". This example is used for cover mu110119 to fit nsd003.

   INPUT
   Projection    TRANSVERSE
   Datum         WGS72 /* THIS IS WGS72
   Zunits        NO
   Units         METERS
   Spheroid      WGS1972 /* THIS IS WGS72
   Xshift        0.0000000000
   Yshift        0.0000000000
   Parameters
   0.99990 /* scale factor at central meridian
   -64 30  0.000 /* longitude of central meridian
    0  0  0.000 /* latitude of origin
   5500000.00000 /* false easting (meters)
   0.00000 /* false northing (meters)
   OUTPUT
   PROJECTION UTM
   UNITS METERS
   DATUM NAR_D THREE
   ZONE 20
   PARAMETERS
   END
   

2. The cover created in 1) above describes the cover in DATUM NAR_D. At this point use PROJECTDEFINE to update the datum to "DATUM NAD83".
3. PROJECT from "DATUM NAD83" to "DATUM NAD27" (uses NADCON method). eg. projection file

   INPUT
   PROJECTION UTM
   UNITS METERS
   DATUM nad83
   ZONE 20
   PARAMETERS
   output
   projection utm
   units meters
   zone 20
   datum nad27
   parameters
   END
   

Simply reverse the steps to convert from NAD27 to WGS84. This two step method takes advantage of the fact that the most accurate way to convert between NAD83 and NAD27 is the NADCON method. It's possible to convert directly to and from WGS84 and NAD27 (using the equivalent datum NAS_C) but not as accurate.

The steps above describe changing datum only and it is recommended that you change the datum first then change map units when the datum change has been completed. Remember to build cover when complete.

Source: Stan Alward / ESRI
Contact: Head, CanSIS