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I've got the following (2D!) situation of overlapping polygons:

enter image description here

I would like to have the polygon border dissolved such that I have only one polygon left. I thought this was a definitive case for the dissolve tool, but somehow the result is bizarre:

enter image description here

What did I do wrong? I haven't chosen the attribute to dissolve on so I thought then all polygons are dissolved and should bring the desired result. Can anybody help?

EDIT: A clipped version of the data can be found here:

https://www.dropbox.com/s/xm9ykgnrbleutgr/extracted_area.zip?dl=0

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3 Answers 3

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I too encountered the bug, here's a workaround:

  1. Do the Merge without the "keep boundaries" that returns the weird result.
  2. Split you source data to individual features
  3. Perform Intersect on the 2 resulting layers
  4. merge the result of the dissolve with the result of the intersect
  5. Final dissolve to create required layer enter image description here

If you need to save attributes of the original data, make sure you retain the fields you need during dissolve operations

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  • +1 for the investigative work, I started going along that line by trying the merge the dissolved with the source, it gave me even weirder results. good effort here.
    – yanes
    Commented Dec 13, 2015 at 22:10
  • Okay, this is a really nice workaround! The only problem I'm struggling with is now that I do not only have two polygons, but over 600...and it is thus not easy to do the Intersect....any ideas how to fix that?
    – countryman
    Commented Dec 16, 2015 at 9:09
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Try it with the keep boundaries box un-ticked. I am assuming that you are using the polygon dissolve field. enter image description here

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  • This yields the same result, unfortunately!
    – countryman
    Commented Dec 10, 2015 at 16:30
  • What is your data like? do you have polygons within polygons? Can you post the exact parameters you used while doing the dissolve?
    – yanes
    Commented Dec 10, 2015 at 16:31
  • The data is actually the top view of trees that overlap at certain areas. I want to have them a one polygon in order to make spatial queries later on that tell me the canopy cover later on. As the polygons are branches it should not happen that there are polygons within polygons. The parameters are just like on your screenshot - no attribute set for the dissolve
    – countryman
    Commented Dec 12, 2015 at 14:51
  • Would you mind uploading a zipped dataset? you can clip only a portion of your polygon and share it. It is working for me. May be cleaning your data might help (if you have dangling nodes etc.). Do try cleaning and building your polygon. if you can't share your dataset.
    – yanes
    Commented Dec 12, 2015 at 22:48
  • Okay, I added a link to a zipped and clipped version of the data. I tried to clean the data following this question, but not with the desired result...
    – countryman
    Commented Dec 13, 2015 at 7:43
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Okay, so here's one workaround I received from SAGA's developer Olaf Conrad, personally:

He said that overlapping polygons are actually not "allowed" as input for the dissolve tool and SAGA assumes that all polygons layers are "clean" (=without overlaps).

His idea is to

1) Use 'Polygon self-intersection' to get the overlaps as polygons, too

2) Use 'Dissolve' to get the desired result.

enter image description here enter image description here

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  • From the commandline the commands are: saga_cmd shapes_polygons 'Polygon Self-Intersection' -POLYGONS INPUT.shp -INTERSECT tmp.shp ; saga_cmd shapes_polygons 'Polygon Dissolve' -POLYGONS tmp.shp -DISSOLVED OUTPUT.shp Replace INPUT.shp and OUTPUT.shp as appropriate.
    – Alex Hajnal
    Commented Feb 27, 2020 at 6:14

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