[Proj] Help translating NAD83 coordinates to WGS84

Ed McNierney ed at topozone.com
Mon Sep 20 16:38:51 EDT 2004

John -

You're still barking up the wrong tree <g>.  If you want to understand the datum shift issues, you need to take a little more time to understand what you're reading.  The "meter or more" reference is in a paragraph discussing the datum itself, and needs to be interpreted in that sense.  Geodesy is a complex subject, not quickly covered by a bit of reading.

Do you have a published reference stating the upper bound on the positional error of the TIGER/Line address coordinates?  I think you may be in danger of losing sight of the fact that this error is so large it dwarfs almost everything else.  The TIGER data puts my street address over a kilometer away from where my house is!

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242  

-----Original Message-----
From: proj-bounces at xserve.flids.com [mailto:proj-bounces at xserve.flids.com] On Behalf Of John D. Evans
Sent: Monday, September 20, 2004 2:29 PM
To: Proj at xserve.flids.com
Subject: Re: [Proj] Help translating NAD83 coordinates to WGS84

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 
>>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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