[Openspace] More on problems with Contiguity Weights from Dissolved
dlong at ssc.wisc.edu
Thu Sep 8 11:28:29 CDT 2005
Thank you all for your replies to my note on the openspace list about
corrupt files and weight matrices. Here's a brief summary of what I
found upon further exploration.
Kurt Paulson noted that the spatial accuracy in 3.3 is different from
that of ArcGIS 9, thus there's potential for imperfect alignment or
sliver polygons to arise from subsequent overlays with the ArcView 3.3
/In response to Kurt's note I took a more careful look at my results.
Using ArcGIS 9.1 I examined the polygons created from each of my
dissolves and discovered that the ArcGIS dissolve did create a tiny gap
between the dissolved polygon and its neighbors (The gap is literally a
minuscule fraction of a millimeter for my county data but usually
visible at the maximum zoom level). So, technically GeoDa is correct to
recognize it as an island polygon. In my data there was no shift
apparent for the ArcView 3.3. dissolve even at the maximum zoom level.
Looking on ESRI's help pages I found we are not the first to encounter
this problem with ArcGIS dissolves and it was supposed to be fixed with
the 9.1 version.
Al Nucci reported a similar contiguity weights problem with files
created in Arcview, so that may not be a reliable fix. He suggested
using the conversion to Geodatabase and subsequent export of a new
shapefile as way of rebuilding the topology.
/In response to Al's note, I tried converting my corrupt ArcGIS dissolve
file to a GeoDatabase then back to a shapefile and it appears to correct
the contiguity problem. I should note however that, in this instance,
there was a shift (about 4 millimeters) that could be an issue for
subsequent intersect or overlay functions. /
/Just to make sure my original problem was replicable, I repeated the
dissolves in ArcView 3.3 and ArcGIS 9.1 and found the same results as I
reported previously. The contiguity structure for the file created in
ArcView 3.3. looks correct, but the contiguity structure for the file
created in ArcGIS 9.1/ /was not./
I've hung my files online in case anyone wants to look at them or
experiment with them further. The files are located at:
Included in the zip file are the following plus associated weights files:
*Predissolve.shp: *Original shapefile with variable /Dis_fips /which I
used for subsequent dissolves.
*ArcGIS9_dissolve.shp:* Dissolve created in ArcGIS based on /Dis_fips/
*AV33_dissolve.shp:* Dissolve created in ArcView 3.3 based on /Dis_fips/
*Gdb_dissolve.shp:* Created by converting ArcGIS9_dissolve.shp into
geodatabase, then exporting to shapefile using ArcGIS 9.1 with default
Please drop me a note if you have other useful insights.
dlong at ssc.wisc.edu
Applied Population Lab
1450 Linden Drive Rm. 350
Madison, WI 53706
Email: dlong at ssc.wisc.edu
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