[GRASS-user] Varying results - v.select,v.distance,v.what

[Kevin Webb]

At 03:56 PM 11/26/2008, Markus Neteler wrote:
>On Thu, Nov 20, 2008 at 9:03 PM, Kevin Webb <kfw4 at cornell.edu> wrote:
>...
> > v.db.connect -p indicates layer#2 is keyed by the <cat> field yet I don't
> > see a 'cat' field in the v.info output,
> > shouldn't there be a 'cat' column in layer#2 or am I misinterpreting how
> > multiple layers are properly added to a vector?
>
>Yes, a cat column should always be there AFAIK.
>
>Can you put the packages location somewhere with copy.paste commands
>to try? Offlist, if needed for privacy.
>
>Markus


Thank you for your reply Markus.

I can copy.paste the vector from inside grass, or I can create a tar-ball 
of the
original source data for testing. Let me know your preference.

I suspect the phenomenon I am experiencing has something to do with using
v.external to do the import and subsequent GRASS interaction with the OGR 
driver,
although there is a strong possibility it is due to the fact that I am new
to GRASS and GIS in general and I have done something improperly. As time
allows, I am attempting to step through the v.external code using gdb to 
see if I
can make any discoveries.

--Background Information--
My primary interest in GRASS is to use its command-line/scripting interface for
bulk data extraction - data will be extracted from approximately 1,400 
layers to be
used for multivariate statistical modelling of bird abundance and 
environmental
issues.

You might wonder why I am using v.external as opposed to v.in.ogr to import 
data
into GRASS. I have executed comparison tests using 65,000 points on layers 
imported
using v.in.ogr and v.external, and the results from the v.external vectors 
more closely
match results from the ESRI arcgisscripting interface than do the results 
from the v.in.ogr
vectors. The ESRI results are NOT any kind of benchmark, but they give me 
values I can
use for comparison. v.external point-in-polygon comparisons give me a .005 
variation
when compared to the same run from ESRI, and v.in.ogr point-in-polygon 
comparisons
yield a variation > .01. I am biased towards using v.external as the import 
mechanism
based on the smaller delta.

Your assistance is greatly appreciated.


KFW




-----------------------------------------
Kevin Webb
Programmer/Analyst
Cornell Lab of Ornithology
159 Sapsucker Woods Rd.
Ithaca, NY 14850
Tel: 607.254.2103
Fax: 607.254.2415
kfw4 at cornell.edu
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