HowTo: Determine which NAD_1983_To_WGS_1984 transformation to use

Article ID:	24159
Software:	ArcGIS - ArcEditor 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3 ArcGIS - ArcInfo 8.0.1, 8.0.2, 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3 ArcGIS - ArcView 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3
Platforms:	N/A

Summary

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 and Pacific (for Hawai'i, etc.) 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 are increasingly different.

Instructions provided help in determining the correct datum transformation to use when transforming data between the NAD 1983 and the WGS 1984 datums.

Procedure

NAD_1983_To_WGS_1984_1
Published accuracy from EPSG is 2 meters. This transformation applies to the entire North American continent. Accuracy of the transformation varies, with greater accuracy at southern latitudes, and less accuracy at more northern latitudes with maximum offset of 2 meters.

This transformation uses the Geocentric Translation method, with the transformation's parameters (dx, dy, and dz) all equal to zeroes. This transformation treats the NAD 1983 and WGS 1984 datums as though they are equivalent.

NAD_1983_To_WGS_1984_2
Calculated by the U. S. Defence Mapping Agency (DMA), now known as the National Geospatial Intelligence Agency (NGA) for the Aleutian islands. Accuracy is listed by EPSG at +/-8 m.

NAD_1983_To_WGS_1984_3
Calculated by the NGA for Hawai'i. Accuracy is listed by EPSG at +/-4 m.

NAD_1983_To_WGS_1984_4
Formerly applied within the 48 contiguous states, but is superseded by _5. This transformation method should no longer be used.

NAD_1983_To_WGS_1984_5
Transformation parameters calculated by the U. S. National Geodetic Survey (NGS) using CORS stations, and ties WGS 1984 to ITRF96. Accuracy according to EPSG is +/- 1 meter.

NAD_1983_To_WGS_1984_6, _7, and _8
Canadian NTv2 transformations, for the Quebec, Saskatchewan and Alberta provinces, respectively.
Each of these datum transformations can be used for the specified area, and arguments can be made for the application of each transformation. One of the most important considerations 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 is recorded in the metadata.
The European Petroleum Survey Group (EPSG) database can be downloaded at the similarly titled link in Related Information. This database is a free download, and is updated frequently. This database includes information on the source of datum transformation parameters, and in many cases includes the accuracy of the transformation from the transformation source.
ArcGIS Desktop version 9.2 uses information from version 6.10.2 of the database. Version 9.3 uses information from version 6.13.
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.

Related Information

Projection Basics: What the GIS professional needs to know
The following concepts are fundamental to understanding the use of map projections in ArcGIS. 1. Coordinate systems, also known as map projections, are arbitrary designations for spatial data. Their purpose is to provide a common basis for comm...
Select the correct geographic (datum) transformation when projecting between datums
It is necessary to specify a geographic (datum) transformation when using the ArcToolBox Project Wizard (ArcGIS version 8.x) or Project Tool (ArcGIS version 9.x) to project shapefiles or geodatabase feature classes between different geographic coo...
European Petroleum Survey Group (EPSG)

Created: 12/31/2002
Last Modified: 9/22/2008