QGIS – Merging and Dissolving Adjacent Polygons to Sum Attributes

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I have many polygons that are adjacent to each other, and others overlap slightly. I need to merge them and sum up their attribute (Area per class). I attach an image of my problem (Screenshot of my merged polygons. Each color represents a landcover class)

I am able to merge the polygons using the function "merge Layers" from the MMQGIS plugin. But the adjancent polygons of the same type are not dissolved, nor their Area summed up.

I tried already using the function Dissolve (per Area) and doesn´t work either.

I am beginner in QGIS. I am working in QGIS 2.12.3 in a PC.

Best Answer

Thanks to the comment of @Ed Rollason I used the Topology checker to understand where was my problem.

Ok, as I said, my layers overlap. But they overlap like a zipper (one on top of each other). After trying many tools (union, difference, merge, dissolve) without success, I found a video that explains the v.clean tool of the Process Toolbox.

In QGIS 2.12.3, go to Processing > Toolbox > GRASS Commands > Vector. I tried v.clean tool like in the video, but I had some geometry errors. Therefore, I used v.dissolve, tolerance = 1 and afterwards the Dissolve tool for a final clean up of the image.

Regarding the attributes, I have the total area of each class.

Link to the video: https://www.youtube.com/watch?v=t1ChPvsC4bU

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After hours of trial and error, I found a solution that I answered here: How to merge and dissolve adjacent polygons and sum up their attributes in QGIS?

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Are you aware that the output from r.clump does not have the depths? Each clump is given a unique category value, so you no longer have the cell depths. You can get the area of each clump, but not the total volume.

If I understand correctly your goal, you should convert the "clumps" to a polygon vector, then use v.rast.stats to get the total depth for each clump area. i.e.:

v.rast.stats map=clumps raster=depths column_pref=depth method=sum,minimum,maximum,range

This will add four new columns to the area vector of "clumps" with total volume, min and max depths, and the range of depths.

I'd also suggest you try to do this procedure directly in GRASS, rather than thru the processing toolbox in QGIS.

Best Answer

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