This is a simple geoprocessing tool that extracts adjacency information from a layer of polygons. The output is a polyline layer with one record for each border between each pair of neighboring polygons.

Download v. 0.1.

After downloading and installing, you can right-click on a new toolbox, choose "Add -> Tool", and select Boundary Generator under the "MWSW Geoprocessing Tools" group. You can also find MWSW Boundary Generator on the "Index" toolbox view.

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Project status (alpha):

Generating boundaries works; initial spot checks are giving correct answers. Some of the Geoprocessing UI behavior isn't quite finished -- e.g., you must specify the output location yourself; the tool gives an erroneous validation error if you try to run it on a layer that isn't in your TOC. (Expect these to be fixed soon.) The tool hasn't really been tested in a model builder / python scripting framework yet, and needs some adjustments to work well there. Project rationale:

It is a good demonstration project for custom programming geoprocessing tools in C#, as well as simple computational geometry & spatial indexing techniques. It is robust and geometry based; the other extensions I've come across for doing the same thing simply searched for line segments with identical endpoints (and would thus fail with partial overlaps, T junctions, numeric precision issues, and the like) or used polygon-polygon intersection to come up with polygon adjacency lists (but not accurate border lines), as described in this old article. This functionality is available without an ArcInfo license. (If you do have ArcInfo, you can use the offical Polygon To Line tool. I would be curious to see the results of a side-by-side comparison from someone with a license...) If you are still using ArcView 3.x, my (dated and not well-tested; beware!) Avenue script for doing the same thing is available at http://arcscripts.esri.com/details.asp?dbid=12786.