[GRASSLIST:5067] Re: v.proj datum errors

Richard Greenwood Rich at GreenwoodMap.com
Mon Nov 25 18:01:37 EST 2002

GRASS 5.0.0 pre4 (and probably earlier) was "hardwired" to read the state27 
definitions when you created a new location. I don't know if this has been 
fixed. Eric Miller suggested renaming you grass5/etc/state27 file and the 
creating a symbolic link to the state27->state83 so that when you created a 
new location it would read the values from state83. So you might look in 
your PROJ_INFO files and see what values got used.

Also - I don't think datum transformations are supported. Somebody please 
correct me if I'm wrong. In my area (western wyoming) there is only about 1 
second of rotation between NAD27 and NAD83 so I have been able to manually 
edit the headers for raster files and get a workable datum conversion for a 
small area (a DRG).


At 05:42 PM 11/25/2002 -0500, you wrote:
>Grass 5.0.0
>I am having trouble projecting vector data to a new projection/datum.
>I have North Carolina state plane (NCSP) NAD83 Meters raster aerial photo 
>I have NCSP NAD83 Feet road data. I v.proj the road data and it appears fine.
>(The roads overlay the roads on the raster.)
>I have NCSP NAD27 feet data of Cadastral (Property).  When I v.proj this 
>data to
>NCSP NAD83 Meters, it appears about 9700 Meters west of the NAD83 data (using
>d.measure).  When I v.proj UTM zone 18 NAD27 Meters Hydrography data, it 
>at the same relative position as the other NAD27 data (the Cadastral data).
>In other words, the NAD83 data lines up. The NAD27 data lines up with 
>NAD27 data.
>But NAD27 does not line up with NAD 83.
>It looks like the projection is occurring correctly but the datum 
>transformation is not.
>I have carefully set up the proj_info and proj_units files.  I even copied 
>over Eric Miller's
>state plane files and tried those.  Same results. (see 
>The difference is suspiciously similar to the difference in false easting 
>between NCSP NAD83 and
>Projecting from the NAD83 M to NAD27 Ft (the reverse of above) gives 
>similar results
>with the NAD83 data appearing 9700 meters east of the NAD27 data
>Am I doing something wrong?  Has anyone else had a similar problem?
>John Gillette

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