[GRASSLIST:5068] Re: v.proj datum errors

roger roger at spinn.net
Mon Nov 25 18:23:20 EST 2002

I recently used v.proj to reproject data from New Mexico
State Plane Central Zone NAD83 to UTM zone 13 NAD27.  My
only problem was in getting the state plane location set up
correctly.  I had to look up the specifications for the
state plane projection and use that to set up the GRASS
location.  v.proj worked correctly once the location was
specified correctly.

Roger Miller

> Grass 5.0.0
> spar-sun-solaris2.7
> 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  NCSP NAD27.
> 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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