Getting my NAD's straight!

[GHATCHER]

Date sent:  2-SEP-1992 12:19:41 
Hi Grass users: 

About a week ago I asked about data sets which were referenced to Clark66
(NAD27) being transformed to wgs84 (essentially NAD83). From the response
I received (or lack thereof) I am assuming there must be some others out
there confused by this issue. So here is an answer to my question.

I have done some investigation into this problem and have found that the
conversion is not a trivial matter but IT IS IMPORTANT for high resolution
data since according to NOAA Technical Memorandum NOS NGS-50 the datum
differences can be in excess of 100m in amplitude.  Not only that, but the
offset is not constant over a given region with local distortions on the
order of 10m.

So what can be done? A program written by people from the National Geodetic
Survey (NGS) called NADCON (North American Datum CONversion) is available
which will do just the conversion I'm talking about (in either direction).
NADCON calculates conversion based on a (painfully created) grid of observed
local shift values and does simple interpolation for points not falling
exactly on a node of the  grid. Using this program the NGS claims an
accuracy of .15 - .5 meters for the conterminous United States. NADCON
results meet National  Mapping Accuracy Standards.

To get a copy of this program:

National Geodetic Survey
NOAA (N/CG174)
Rockville MD 20852
Telephone Number: (301) 443-8631

Caveats:
It's a PC program. It's written in FORTRAN. It only handles a limited number
of data formats.

Where I got this information:

NOAA Technical Memorandum NOS NGS-50
available from:
Same source mentioned above.


PS. I also asked grassu-l if anyone knew what dx,dy,dz were about in
m.datum.shift and nobody responded. I've since decided that they must be
the Molodensky transformation parameters - Not for those with weak stomachs!

I hope this helps someone, clears up some confusion, or at least helps
someone get to sleep!

Gerry Hatcher                   

*** Soon entering the job market with an MS in OCEAN ENGINEERING.  ***
*** PLEASE INQUIRE for RESUME, REFERENCES, and more INFORMATION    ***

Thesis: Geographic Information Systems as a Data Management Tool for
        Sea Floor Mapping.
___________________________________________________________________________
Ocean Mapping Development Center     | gerry at omdc.gso.uri.edu {Internet}
University of Rhode Island           |      voice (401)-792-6768      
Narragansett, RI 02882               |       fax (401)-792-6849
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