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 NADCONmethod). 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.