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My problem is quite similar to those of title "qgis merge polygons does not merge on snapped edges" and "Merging polygons". I'm merging several polygon features (within one layer), all adjacent with each other, the sum of which cover a greater area, which I need. To do this I'm either using Vector > Geoprocessing > Dissolve or the tool "merge selected features" while in edit mode. Both result in a new shapefile with only one feautre (attribute's don't matter now), which is OK for me. However the new feature contains 3 kind's of "mistakes". 1. Rings (inside the polygon area) 2. Rings with zero area (also inside the polygon area) 3. "Peninsula"-s (at the boundary of the polygon).

Three types of "mistake"-s

The first two I can resolve manually by deleting the nodes one by one (somewhat cumbersome!). This is true for the third type of error, too, but to resolve the third type of error, I should go manually over the whole boundary of the new feature, which is far too cumbersome.

How can I automatically get rid of these three types of "mistakes"? NO modification of other than the above vertices should be guaranteed! Part of the solution would be to delete all "duplicate" verticies, but this is not enough (see joining point of the "peninsula").

Using QGIS 2.6.1 Brighton on Windows 7 64.

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Looks like you got topological errors in your data. GRASS has a cleaning tool called 'v.clean'.

AFAIK this tool has a bigger scope of functions when used directly from GRASS than from QGIS.

There are also options for special cases like yours (dangles and bridges).

v.clean in QGIS processing tool box

It is kind of an advanced technique to use GRASS directly but take a look in its documentation and decide for yourself:

GRASS documentation - v.clean

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