[GRASSLIST:2438] Re: vector/sites mismatch

[Steven Stoddard]

On Jan 31, 2004, at 5:14 AM, Hamish wrote:

> On Fri, 30 Jan 2004 15:36:20 -0600
> Steven Stoddard <sstoddar at 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
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