[Openspace] dissolve and contiguity
koschins at uiuc.edu
Thu Sep 9 17:07:26 CDT 2004
After zooming in to your shapefile, you can see that several
polygons are surrounded by polygon "slivers" or gaps between
polygons after the dissolve process (we have seen the same
problem with other dissolved shapefiles).
To get a shapefile that can be used to create a working
weights matrix, use the CLEAN command in ArcInfo before you
dissolve. Check your shapefile after the clean operation to
make sure it worked correctly (sometimes there are problems
with the operation; note that you can adjust your tolerance
level). If the clean command worked, then perform the
dissolve function, which should leave you without any
slivers. At that point, you can create a correct weights
matrix in GeoDa (we tested this successfully in practice).
---- Original message ----
>Date: Wed, 8 Sep 2004 16:05:04 -0700
>From: "Kevin Drakulich" <kmd3 at u.washington.edu>
>Subject: [Openspace] dissolve and contiguity
>To: <openspace at agec221.agecon.uiuc.edu>
>I know a similar question has been posted to this list
before by Mike
>Sobczak. I believe there was no answer to that, but I'm
hoping someone may
>have one now.
>Like Mike, I began with a Tiger shapefile, and
>in ArcMap 8.2 to aggregate smaller polygons into bigger
ones. For me, the
>move was from census tracts to "neighborhoods" (collections
>tracts) within a city.
>Also like Mike, when I created a contiguity-based weight
file in GeoDA,
>there are lots of mistakes in the file. For some
neighborhoods there are no
>neighbors (which is clearly not the case), and for others,
>listed are clearly wrong (if you look into the .GAL file).
>None of these problems exist if you bring in the original
TIGER files of
>census tracts pre-dissolved.
>Has anyone else seen this problem or know what might be
>Kevin M. Drakulich
>University of Washington, Seattle
>Openspace mailing list
>Openspace at sal.agecon.uiuc.edu
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