[GRASSLIST:5066] v.proj datum errors

John Gillette JGillette at rfmd.com
Mon Nov 25 17:42:42 EST 2002

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 data.
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 appears
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 http://pweb.jps.net/~egm2/grass/STP/index.html)
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
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