I have 2 related questions that hopefully someone can cast some insight on. I am not sure if they are a result of a naive oversight or not, but ...
I have (attached) a sites file and a vector file both created in Arc -- the vector file is supposedly a polygon representation of the sites file (green crosses), but pulling these into GRASS, I find that the polygons and the sites don't correspond. There seems to be a semi-consistent N-S displacement of about 200 m, though not in all cases. Is this perhaps a result of a misprojection? I have this same problem with a SPOT image.
What would be the best way to correct the mismatch and ensure that the maps are lined up properly?
Similar to that question, I did sub-sampling of a set of sites using d.sites.pg -- which gave me the new subsets as separate site files but they do not align with the original sites! There is a 205m displacement due south. ???
What am I missing here?
Thanks for any help/advice in advance,
Steve
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On Jan 31, 2004, at 5:14 AM, Hamish wrote:
On Fri, 30 Jan 2004 15:36:20 -0600
Steven Stoddard <sstoddar@uiuc.edu> wrote:
I have 2 related questions that hopefully someone can cast some
insight on. I am not sure if they are a result of a naive oversight
or not, but ...
I have (attached) a sites file and a vector file both created in Arc
-- the vector file is supposedly a polygon representation of the sites
file (green crosses), but pulling these into GRASS, I find that the
polygons and the sites don't correspond. There seems to be a
semi-consistent N-S displacement of about 200 m, though not in all
cases. Is this perhaps a result of a misprojection? I have this same
problem with a SPOT image.
What would be the best way to correct the mismatch and ensure that the
maps are lined up properly?
Similar to that question, I did sub-sampling of a set of sites using
d.sites.pg -- which gave me the new subsets as separate site files but
they do not align with the original sites! There is a 205m
displacement due south. ???
What am I missing here?
are you doing a datum transform? 100-200m shifts are a symptom ..
Datum support is mostly missing in GRASS 5.0.x, you'd have to try 5.3 or 5.7.
But why one should work and the other not???
I take it you are importing the shape file into its native projection
and using s.proj and v.proj? Can you do d.where and d.what.sites on the
points and compare to the positions in Arc to see if it is an import or
projection problem?
Hi. Thanks for the response.
To address some of your questions: I actually just imported the shapefile -- unfortunately there was no associated metadata, so I assumed that the projection and datum would be the same as a number of other shapefiles I received from the same source for that location. I will do the comparison with ARC when I have access, as you suggested.
The SPOT image I have the mismatch problem with was imported with r.in.gdal. I will try importing with 5.7 into its own location and then using r.proj ...
But--what about the sites mismatch caused by creating sitefiles with d.site.pg? I don't understand why the input and output maps don't line up when everything was done in the same location ...
regards,
Hamish
Thanks,
Steve
---
Steven T. Stoddard
Program in Ecology and Evolutionary Biology
University of Illinois at Urbana-Champaign
302 Shelford Vivarium
606 E. Healy Ave
Champaign, IL 61820
(217) 333-2235
(847) 433-6396
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