How To: Determine which NAD_1983_To_WGS_1984 transformation to use
When originally defined, North American Datum 1983 (NAD 1983) and World Geodetic System 1984 (WGS 1984) were equivalent. In order to minimize coordinate changes, NAD 1983 is tied to the North American, Pacific (for Hawaii, Guam, etc.), and Marianas tectonic plates. WGS 1984 is tied to the International Terrestrial Reference System (ITRF) which is independent of the tectonic plates. Over time, the two coordinate systems have become increasingly different.
The instructions provided should help in determining the correct geographic/datum transformation to use when transforming data between NAD 1983 and WGS 1984.
The accuracy of the transformations listed in this article are in meters. The accuracy values have been calculated by the organization that created each transformation. These values are provided as a convenience to users, but are not verified by Esri.
Here is the list of supported geographic/datum transformations between NAD 1983 and WGS 1984 at ArcGIS Desktop 10.3.1. Certain transformations may not be available at earlier version of the software.
One of the most important considerations in selecting a geographic transformation is consistency: using the same transformation each time to transform between these two geographic coordinate systems (datums). When using the Project Tool, the transformation method applied is recorded in the metadata.
The EPSG Geodetic Parameter Dataset can be downloaded from the similarly titled link in the Related Information section below. This database is a free download and is updated frequently. This dataset includes information on the source of geographic/datum transformation parameters and may include the accuracy of the transformation from the transformation source.
Note: ArcGIS Desktop version 9.2 uses information from version 6.10.2 of the database. Version 9.3 uses information from version 6.13. ArcGIS Desktop version 10.0 uses information from version 7.4. Version 10.3.1 utilizes information from 8.6.2.
Note: Information from new versions of the EPSG database are not automatically added to the Esri projection engine. New items are added when requested by users.
The difference between the GRS 1980 spheroid, the basis for the NAD 1983 datum, and the WGS 1984 spheroid is 0.0001 meters in the length of the semi-minor axis - the distance between the geodetic center of the earth and the North Pole. The semi-minor axis for GRS 1980 is 6356752.3141 meters. This axis for WGS 1984 is 6356752.3142 meters, while the semi-major axis both spheroids measures 6378137 meters.