[GRASSLIST:1826] Re: r.proj datum conversions

Richard Greenwood Rich at GreenwoodMap.com
Wed Nov 19 18:59:46 EST 2003

>On Wed, 19 Nov 2003, Richard Greenwood wrote:
> > Hi Paul,
> >
> > In response to the subject "NAD83 datum" you said that 5.0.x does transform
> > between nad27 and nad83. Although I did not state it in my earlier email
> > (subject "r.proj datum conversions"), I converted from UTM NAD27 to state
> > plane NAD83, but saw no datum transformation. I was using 5.0.3 r.proj.
> >
> > So I am confused because in response to my general question, you said that
> > datum transformation was not supported in 5.0.x, but do I understand
> > correctly that nad27 <=> nad83 transformations are available in 5.0.x?
>Yes that is correct, although there were some bugs involving latlong locations
>(now fixed in 5.0.3) but UTM to state plane should be OK. You should
>make sure the PROJ_INFO files in the PERMANENT mapset directory for each
>location have a line
>datum: nad27
>datum: nad83
>(note it must all be small letters)---it is OK to edit these files
>manually to set it up---and then r.proj, s.proj or v.proj will print a
>message to the screen confirming that they are doing the datum shifting.
>As far as I can think there shouldn't be anything else you need to do. Let
>me know if it works.

At 04:01 PM 11/19/2003, Paul Kelly wrote:


Perfect, it worked, thanks! I am copying the GRASS list under the original 
subject for continuity of thread.

I was attempting to r.proj from UTM NAD27 to state plane NAD83, however my 
state plane PROJ_INFO lacked a datum and an ellps. (I had created the state 
plane location some time ago, and I forget in what version of GRASS). So 
r.proj was working, but without the datum shift. Adding the lines:
     datum: nad83
     ellps: grs80
to the state plane PROJ_INFO got the datum shift into the transformation 
and the results are excellent.

Thanks a bunch.


Richard W. Greenwood, PLS
Greenwood Mapping, Inc.
Rich <at> GreenwoodMap <dot> com
(307) 733-0203

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