"Merge Selected Features" Tool don't work properly

[qgib]

Author Name: Tomas Cabezudo (Tomas Cabezudo)
Original Redmine Issue: 15555
Affected QGIS version: 2.16.1
Redmine category:geometry

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I found many situations where this tool seems not work properly. I attach a zip file with a shapefile having two adjacent poligons (two pictures showing before and after included too).
Before this, I have verified them with topology checker tool whitout errors.

I select this two adjacent polygons, and when I ejecute this tool, I get a new polygon, but still appears some segments from comun edge in the new polygon..

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• MERGE_ERROR.ZIP (Tomas Cabezudo)

[qgib]

Author Name: Tomas Cabezudo (Tomas Cabezudo)

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• assigned_to_id removed Tomas Cabezudo

[qgib]

Author Name: Jukka Rahkonen (Jukka Rahkonen)

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This is not a bug really but your data is not ready for merging. The two polygons do not share exactly same coordinates. Here is one example:

POINT (197903.09522296838 4796248.478830873)
POINT (197903.09522296768 4796248.478830872)

The difference is in the ninth decimal bet still the vertices are not the same. You must pre-process your data by snapping the vertices to some grid with a reasonable precision. Or then you could make a feature request for making the tool in QGIS to handle vertices with some given tolerance.

You can study your data with a quite useful tool in OpenJUMP Plus. The tool is in Plugins-Topology-Find coverage gaps and Find coverage overlaps in layer. Or you can use the Fence tool and the right-click-Other tools-Vertices In Fence. I do not mean to especially advertise OpenJUMP but I believe that QGIS does not have this kind of tools yet.

[qgib]

Author Name: Tomas Cabezudo (Tomas Cabezudo)

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Thanks Jukka.

Yes, as You remarked me, some coordinates are different (but 0.000000001 mts..). Really, I don't know what is the value of epsilon to consider the same two values in floating point..
I try to use this tool after to establish setting for snap (to vertex, 0.001 map units), but result are the same.
Obviously, this tool works with no tolerance. Then, as you say, perhaps the best idea is to make a feature request for making the same tool in QGis, but adding tolerance value. I think that would be a more robust tool.

[qgib]

Author Name: Giovanni Manghi (@gioman)

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you can fix this kind of situations by using Processing/GRASS/v.clean and the snapping options.

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• resolution was changed from to invalid
• status_id was changed from Open to Closed