TEXT by Dave Doyle, NGS (10 May 2000)
              "NAD-83 vs WGS-84"

Consider the relationship of the North American Datum1983 (NAD 83) 
coordinate system relative to it's relationship to earth-mass-center 
which is approximately 2 meters (3-dimensionally).  

The 2 meter "error" between datums is not really an error, 
but rather a difference in the location of the origin. 

NGS first published the NAD 83 in June, 1986.  NAD 83 was 
the first civilian datum to have it's origin at or near 
earth-mass-center.  Our knowledge of the location of earth 
center was derived from using data from the TRANSIT DOPPLER 
satellite system, which is the system which preceded GPS.  
The accuracy of Doppler positions are approximately 1 meter 
in each component.  Consequently, our knowledge of earth 
center is +/- 2 meters.  The initial solution of WGS 84 also 
used Doppler data and consequently had a +/- 2 meter origin. 

Improvements in GPS and other space-based positioning technologies 
(e.g. Very Long Baseline Interferometry, Satellite Laser Ranging 
etc.) have improved our knowledge of earth center to approximately 
4-5 cm!  These improvements have been the work of the International 
Earth Rotation Service (http://hpiers.obspm.fr/) using data 
submitted by many national surveying agencies and international 
academic institutions.  The IERS publishes these data as part of 
the International Terrestrial Reference System (ITRS). 

In 1994, the Defense Department National Imagery and Mapping Agency 
(NIMA) redefined the origin and orientation of WGS 84 to be 
consistent with the ITRS.  This is commonly referred to as the 
International Terrestrial Reference Frame of 1994 (ITRF94).  
This redefinition means that WGS 84, now more accurately referred 
to as WGS 84 (G873) - see http://www.nima.mil/GandG/pubs.html - 
"TR 8350.2 World Geodetic System 1984 It's Definition and 
Relationship with Local Geodetic Systems" , is now equal to 
ITRF94 +/- 10 cm. 

NAD 83 has had several improvements of the international precision 
using GPS.  These improvements are referred to as the High 
Accuracy Reference Networks (HARN), and Continuously Operating 
Reference Stations (CORS).  Consequently the internal or relative 
relationships of these points in NAD 83 are at the 1-4 cm level.  
However, we have not changed the origin or orientation of  NAD 83 
as has been done with WGS 84 (G873).  Therefore, the "absolute" 
difference between points in NAD 83 and WGS 84 are approximately 
2 meters.  This can be seen by looking at the data sheets for any 
CORS site -http://www.ngs.noaa.gov/CORS/, selecting "Site Metadata", 
then "coordinates" and then selecting any CORS site with positions 
in NAD 83 and ITRF94.   You will notice the positional differences 
are approximately 1 meter in both horizontal and ellipsoid height. 

I hope this answers some of your questions. I would recommend that 
your "colleagues" read the series of articles on reference systems 
published by Dr. Richard Snay and Dr. Tomas Soler in Professional 
Surveyor Magazine , December 1999, February 2000 and March 2000. 

Dave Doyle