Adoption of the North American Datum of 1983 requires that users store and use their map data in a coordinate system based on NAD83. However, many users automated their original data layers using NAD27 and continue to do so, primarily because most source data is collected from maps based on NAD27.
Due to differences in the method for defining each datum as well as variations in distortion, the relationship between the NAD27 and NAD83 datum cannot be defined at every location by a single mathematical equation. A number of methods have been proposed for performing the datum adjustment. The currently accepted method is an error averaging, rubbersheeting approach. A regular grid of control points whose datum shifts are known is used to estimate the shift at other locations.
Two such methods for transforming between NAD27 and NAD83 have been developed by the United States and Canada for use in their respective countries.
United States method (NADCON)
NADCON, an acronym for North American Datum CONversion, is a datum transformation program created by the U.S. National Geodetic Survey (NGS). It is suitable for the United States, Puerto Rico, and the Virgin Islands but only for conversion between the NAD27, the NAD83, the High Precision GPS Network (HPGN) readjustment of NAD83, and selected local island datums discussed below. It has been implemented within the Project tool. A minimum curvature-derived surface based on the National Geodetic Reference System determines the transformation of points and has an approximate accuracy of 0.15 to 0.5 meters. NADCON is the accepted national standard and is the fastest, simplest, and most accurate datum transformation for mapping at scales of 1:200 and smaller.
To transform datums, specify different input and output datums with the DATUM subcommand in a projection file. The following projection file would convert an NAD27 dataset to an NAD83 dataset:
DATUM NAD27 NADCON
DATUM NAD83 NADCON
The following keywords (and associated regional coverage) are valid datum names when using the NADCON method for datum transformation.
||Coterminous United States
||20 to 50
||-63 to -131
||46 to 77
||-128 to 166
||Puerto Rico and the Virgin Islands
||17 to 19
||-64 to -68
||18 to 23
||-154 to -161
||56 to 57
||-169 to -171
||62 to 64
||-168 to -172
||57 to 58
||-169 to -171
||Encompasses all the above-listed areas
||See list of supported states in the following section (Accuracy and uncertainty)
Use OLD_HAWAIIAN and ALASKAN_ISLAND keywords to transform from the old island datums to NAD83. If your data is already in NAD27, do not use these options. NAD27 was not used officially in Hawaii or the Alaskan islands. You cannot convert between OLD_HAWAIIAN or ALASKAN_ISLANDS and NAD27 because NADCON does not have the necessary information. The STATEPLANE projection will not allow you to use the datum keywords OLD_HAWAIIAN or ALASKAN_ISLANDS. If you are converting to or from State Plane, use the NAD27 keyword instead. Here's an example. The input data is actually on the Old Hawaiian datum.
The following statement is from the release notes of NADCON Version 2.1, October 1993:
“The accuracy of the transformation should be viewed with some caution. At the 67 percent confidence level, this method introduces approximately 0.15 m uncertainty within the coterminous United States, 0.50 m uncertainty within Alaska, 0.20 m uncertainty within Hawaii, and 0.50 m uncertainty within Puerto Rico and the Virgin Islands. In areas of sparse geodetic data coverage NADCON may yield less accurate results but seldom in excess of 1.0 m. Transformations between NAD83 and the States/Regions with High Accuracy Reference Networks (HARN) introduce approximately 0.05 m uncertainty. Transformations between old datums (NAD27, Old Hawaiian, Puerto Rico, etc.) and HARN could combine uncertainties (that is, NAD27 to HARN equals 0.15 m + 0.05 m = 0.20 m). In near offshore regions, results will be less accurate but seldom in excess of 5.0 m. Farther offshore, NAD27 was undefined. Therefore, the NADCON computed transformations are extrapolations and no accuracy can be stated.”
State readjustments (HPGN/HARN)
There is an ongoing effort at the state level to readjust the NAD83 datum to a higher level of accuracy using state-of-the-art surveying techniques that were not widely available when the NAD83 datum was being developed. This effort, known as HARN or HPGN, is a cooperative project between the National Geodetic Survey and individual states.
Currently, all states have been resurveyed, but not all of the data has been released to the public. As of September 2000, the grids for 44 states and two territories have been published.
Why y coordinates may seem 200 meters too large
Nearly all published statements about the amount of shift between NAD27 and NAD83 are shifts measured in meters on unprojected (latitude–longitude) data. When two datasets are projected into planar coordinates (such as UTM or State Plane) and the differences between locations in NAD27 and NAD83 are measured, the y coordinate is often approximately 200 meters larger than expected. This is a result of the difference between the size and origin of the two spheroids on which the data is being projected.
To see this for yourself, you can create a coverage of a single arc, running along a meridian from the equator to your point of interest in the coterminous United States. By default, it will be created on the Clarke 1866 spheroid. Now Project
it and change the datum to NAD83. The arc should be approximately 200 meters longer.
Canada’s standard transformation program between NAD27 and NAD83 was developed by the Canadian Geodetic Survey. It is known locally as the National Transformation, and will be referred to in this document and the software as the Canadian National Transformation.
The Canadian National Transformation predicts to within 0.01 m of the actual difference for 74 percent of the cases, and within 0.5 m for 93 percent of the cases.
If you have questions about obtaining a more reliable transformation in major urban centers, or other areas that are densified, please contact ESRI Canada for further details.
46 Gervais Road
Toronto, Ontario M3C 1Y9
The following projection file would convert an NAD27 dataset to an NAD83 dataset in Canada:
DATUM NAD27 CNT
DATUM NAD83 CNT