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I am trying to create Thiessen (Voronoi) polygons, based on points within a defined shapefile extent.
The output should be inclusive to the feature. The output I receive now is the rectangular area around the selected points.

Steps I am using:
1. select points of interest (inside and outside spatial extent)
2. set primary display field to FID (aka:ObjectID)
3. Environment… > Set 'Analysis Mask' to shapefile with feature (in this case, a watershed)

Now here is where the problem starts.
I have a set of instructions for arc 9.3, but I am now operating on ArcGIS 10 (Spatial Analyst installed & activated).
Also, just to clarify, I did scour the inter-webs for some number of hours- Esri support, gis.SE, Professor Google...


9.3.1 version:
Spatial Analyst >> Distance >> Allocation
- set 'Assign To' as point shapefile of selected data
- set cell size

10.0 methods tried:
Analysis >> Create Thiessen Polygons -- Many variations of selected features
-- File locations, both inside and out of file geodatabase

I am open to any solution, using Arc 10.0 and/or python would be ideal. The first image is an example. The output does not need to be exact.


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I may be missing something here, but I just tested on my machine to make sure. Use the Create Thiessen Polygons tool in Analysis Tools>Proximity>Create Thiessen Polygons. Then you specify your analysis mask by clicking on Environments>Processing Extent and select your feature class that has the watershed boundary. This is giving me the output you are looking for I believe, I am using Arcmap 10 as well. Let me know if I missed something here. See image below....

Thiessen Polys

The green is thiessens for the entire dataset, while the blue is theissens with an analysis mask (your watershed) specified.

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That is the exact output that I get as well. I would like it to be bounded by the watershed, not extending outside. Clipping this would produce a visually OK result, but ideally I want these p-gons to represent the processes inside the watershed. – Jeremy J Apr 20 '11 at 16:46
I don't think i was clear. The blue area is a seperate set of polygons that was created using a shapefile as the mask. The extent of the blue thiessen polygons is exactly the same as the extent of the analysis mask. The green thiessen polygons are done using no mask and cover the entire dataset. – badkins Apr 20 '11 at 18:33
My results; My desired results I will be visiting a 9.3.1 box today to see if my previous (functional) methodology works. Stay tuned for updates!! – Jeremy J Apr 20 '11 at 18:43
Why not just build this process in ModelBuilder and throw a "clip" routine at the end so that the results come out the way you want. If automation of the routine is all that is necessary, this seems like an easy way to accomplish the task. – RyanDalton Apr 20 '11 at 19:10
@Ryan Dalton, clipping is what I am trying to avoid here. If it comes down to it, your ModelBuilder suggestion would work great. – Jeremy J Apr 20 '11 at 23:41

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