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I have a layer of regions (25 regions) and a layer with thousands of polygons. I want to dissolve this into one polygon for each region (so I can calculate the exact geometry). I used the dissolve tool of ArcGIS, but using this dissolves it into one layer. Is there a trick for this? Can't seem to see it in the dissolve tool itself.

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    What Dissolve tool are you using? The one from QGIS or the one from ArcGIS for Desktop or ...? Whichever it is please edit your question to say and include the precise parameters that you are using. – PolyGeo Oct 15 '15 at 10:05
  • Assuming your region layer has attributes with the name of the region, you could do a spatial join (or the Join attributes by location tool in QGIS) using both layers. Your output should contain your polygon features with their associated region. Now you could run dissolve using the region field which should give you a single polygon per region. – Joseph Oct 15 '15 at 10:25
  • I though about this. However, some polygons are inside two or more regions. I don't want this polygon to assign to 1 region, but split it into several. I want to calculate the total geometry of all polygons inside the region. So I have to split some polygons – user30058 Oct 15 '15 at 10:42
  • You can try using the Clip tool first which should split your polygons with respect to the region they reside in – Joseph Oct 15 '15 at 11:14
  • Intersect them, dissolve using big polygon IDs – FelixIP Oct 15 '15 at 23:50
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You should be able to do that by using two tools, Dissolve and Split:

  1. First of all, add field for region in polygon attribute table. This field must be text-type.
  2. After that, dissolve your polygon layer but remember to check Create multipart features (it is optional) but by default it is checked.
  3. Finally, split same region in different layer by Split(Analysis) tools in Arctoolbox in specific Workspace. Split Field in Split(Analysis) is the field region you had created already.
  • I edited your post but feel free to roll-back the edits if you dislike it :) – Joseph Oct 15 '15 at 13:24
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You can also try the Union tool to combine your region and polygon layers (it will split polygons that lay across region lines and assign the appropriate attributes to each piece although area fields will have to be recalculated) and then dissolve on the region.

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