[Proj] Help translating NAD83 coordinates to WGS84

John D. Evans jdevans at gst.com
Mon Sep 20 14:29:11 EDT 2004

Thanks Norm,

for your speedy reply. That settles it then! ... except that a couple of
paragraphs earlier in that reference (22.2, 22.2.1), I learn that the 
actual differences may be "a meter or more. Although this might be 
disturbing to some," it's only due to the compound uncertainties 
involved in defining the two datums ... or in surveying real-world 
positions ... Or something.

Obviously I don't understand the explanation, because I do find the
"meter or more" statements disturbing :-). Any thoughts?

Norman Vine wrote:
> John D. Evans writes:
>>Can someone point me to an authoritative, published (pref. online)
>>reference that states an upper bound on the difference between NAD83 and
>>WGS84 over North America?
> NOAA Professional Paper NOS 2, Article 22. Page 249
> Charles R. Schwarz, Editor
> National Geodetic Survey
> Charting and Geodetic Services
> National Ocean Service
> Rockville, MD 20852
> December 1989
> online @
> http://www.mentorsoftwareinc.com/resource/Nad83.htm
> 22.2.4 Computational Differences
> There are some differences between NAD 83 and WGS 84 which may arise because of approximations made in a particular method of
> computing coordinates. For most applications, the effect of these approximations is considerably smaller than the effect of
> observational errors. These differences are important only if one is testing the accuracy of a set of equations or a method of
> computing coordinates.
> One such set of approximations concerns the different ellipsoids used for NAD 83 and WGS 84. This difference has no effect on the
> three-dimensional coordinates of a point computed by satellite surveying. If such a set of three-dimensional Cartesian coordinates
> is converted to latitude and longitude using the two coordinate systems, there would be no difference in the longitudes, and the
> latitude difference would be
> which reaches a maximum value of 0.000003 second of arc (or 0.0001 meter) at a latitude of 45 degrees. It is assumed that most users
> will ignore this very small difference.
> Norman


   - John D. Evans, Ph.D. <jdevans at gst.com>
     Global Science & Technology, Inc. (GST)
         Geospatial Interoperability Group
     1-240-542-1133  /  1-301-286-0803

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