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.
- 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
- 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".
- 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.
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