I am new to 10.0 and have never used the thiessen polygon tool — i have a set of points and would like to create a thiessen polygon; what do i need to input (i.e. what type of input (.shp etc)?
[GIS] Thiessen polygon input help
arcgis-10.0voronoi-thiessen
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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....
The green is thiessens for the entire dataset, while the blue is theissens with an analysis mask (your watershed) specified.
Several comments have good suggestions to check for duplicate points stacked atop each other and points with no or bad geometry. However the statement in the question that the tool worked partially cued me to take a look at the help file where I noticed the following:
This tool may produce unexpected results with data in a geographic coordinate system since the Delaunay triangulation method used by the tool works best with data in a projected coordinate system.
Based on that I questioned which coordinate system you were working in. In your investigations it appears you found that your points weren't projected at all which would probably lead to similar behavior. Apparently getting the points into a proper projection solved the issue with the tool.
I do want to point out a couple of things regarding projections in ArcGIS, just for clarification:
- From your comments, the original points were in shapefile format. There should have been a .prj file among the files comprising the shapefile. If it was missing or damaged, that would lead to the issue and warning you got when you tried to add them to a map.
- The Dataframe (with a default name of Layers) can have a projection set in its properties. This can differ from the projection of the individual datasets added to the dataframe. ArcGIS reprojects those individual layers/datasets on the fly to whatever projection the dataframe is using if it can. It can't if there isn't one defined in the first place - it knows what to project to because of what you set the dataframe to, but it doesn't know what to project from because the data doesn't say. It's like saying 'can you give me 532 in meters?' 532 what?
- Projections also involve Datums, and ArcGIS handles them in separate steps. If the datum between two projections doesn't match, you must specify an appropriate transformation to use when converting between them or your points may end up in entirely the wrong place. You can't just pick the projection you want to use, you must specify both (though it's in the same dialog box).
- Geodatabases themselves, be they file or personal, don't have projections/coordinate systems. It's the individual feature classes (or feature datasets) inside the geodatabase that do, and you can have several feature classes with different projections/coordinate systems in the same geodatabase.
- You should be cautious about just 'defining' the CRS of an unknown dataset (which is essentially what you did with the export). If you don't know what it was really in, you have to check that it's coming in 'close enough' to where it really should be based on other known reference data. You've given it a projection now, but it may not be the right one (just the one you want to use). The difference may be fractions of a meter or hundreds of kilometers. For your purposes it may not matter at all where the points are except relative to each other, or your accuracy requirements may deem whatever projection you used to be 'close enough'.
Best Answer
Here is the ESRI help document for the Create Thiessen Polygon tool: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00080000001m000000.htm
You'll need a license for ArcInfo.
The input must be a point feature class (shapefile, geodatabase layer, etc).