[GRASSLIST:4218] NAD27 vs NAD83 SDTS files

Victor Wren vwren at timension.com
Mon Aug 5 17:39:22 EDT 2002

I hope somebody out there can shed some light on this.  I'm doing 
some work in north-central Colorado, and need accurate road and 
elevation data.  I downloaded the SDTS files for the areas I need, but 
they seem to be straddling versions.  Using v.in.sdts, I imported three 
versions of the roadmaps for the quad I'm looking at.  Two of them lay 
in correctly with the DEM SDTS map (imported with r.in.gdal), but the 
third drops in about 300 feet north.  Unfortunately, in back-checking 
the coordinates to the paper maps, the northerly one is the only one in 
the correct location, which suggests that there is the same coordinate 
shift in r.in.gdal and v.in.sdts WITH certain versions of the SDTS files.

What seems to be happening (maybe) is that r.in.gdal assumes that 
the SDTS file is using the same NAD as the working region (in my 
case NAD83).  I say this, because I have a third set of datapoints 
which are done as State Plane NAD83, but if I convert them to UTM 
NAD27, they drop onto the (already imported) DEM in the correct 
location (as near as my eyeballs can figure).

At this point, my brain is starting to hurt, so I can't say whether this is 
the wrong coordinates and the correct correlation to the DEM (likely -- 
and adequate, since all I needed was elevations for the sites), or 
whether the DEM and the sites are all wrong in the same direction 
(actually, that's the same thing, isn't it?).

I'd love to know if anybody has encountered this shift, and what was 
done to correct it.  Incidentally, I encountered the exact same 
coordinate shift looking at the SDTS files using the V3.7 DLG viewer 
from the USGS, but NOT with DLGV32 Pro (the later version), so I 
suspect it's a problem with incompatible SDTS versions (is there a 
later version of v.in.sdts and r.in.gdal or libgdal.1.1.so)?  I'm using 
Grass 5.0 Pre 5  I notice there's a  libgdal.1.1.7.so, but I'm not sure if it 
fixes my problem, since I can't find a changelog (I'm also not 100% 
sure what version I'm using, since I'll have to reboot this computer to 
look at it in the other partition).

Thanks much!
Victor Wren

Victor Wren
Timension Inc.
1350 C Pear Ave
Mountain View CA 94043
(650) 564-9397
Fax: (650) 564-9398
Opinions stated in this letter are not necessarily
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