ftools dissolve (on master) produces empty output if input has invalid geometries

[qgib]

Author Name: Michele Mor (@Michele100)
Original Redmine Issue: 13752
Affected QGIS version: master
Redmine category:processing/qgis

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New description, see #21779 (comment)

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Old description:

Hi.
Using QGIS 2.8.3 Wien I use Vector/Geoprocessing Tools/Dissolve to merge together polygons.
Unfortunately my new layer has an hole (missing polygons) where my original layer has polygons.
I am not able to add a screenshot within this text, so I attached it as a file.

The original layer is Units, the dissolved layer is sectors.
To dissolve the original layer I have used field PCSECTOR.

I have attached 2 zip files.

Note: using QGIS 2.12 dissolve does not work!

Thanks.
Michele

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• 2015-11-04_10_39_04-QGIS_2.8.3-Wien.png (Michele Mor)
• Units.zip (Michele Mor)
• Sectors.zip (Michele Mor)

[qgib]

Author Name: Giovanni Manghi (@gioman)

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The real issue in now another: on QGIS > 2.8.3 with the provided sample data dissolve produces an empty output.

The problem with the "hole" on qgis <= 2.8.3 is somehow expected: the input file has invalid geometries (you can see it clearly also in the attached screenshot) and ftools dissolve (as any other tool in ftools) is fragile in this cases. Anyway in QGIS we have robust alternatives, try the grass or saga dissolve in the Processing toolbox and you'll get the expected results.

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• crashes_corrupts_data was changed from 0 to 1
• subject was changed from QGIS Dissolve creates holes (polygons disappear) to ftools dissolve (on master) produces empty output if input has invalid geometries
• category_id was configured as 44
• priority_id was changed from Normal to Severe/Regression
• fixed_version_id was configured as Future Release - High Priority
• version was changed from 2.8.3 to master

[qgib]

Author Name: Michele Mor (@Michele100)

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Hi Giovanni,

I have tried with GRASS and it had the same issue.
It worked with SAGA.
I am confused thou, because I cannot understand what is wrong with that layer.
I am not trying to modify any polygons, just to merge some if they have the same sector and if they are adjacent, otherwise they should simply become "islands" with the same name.

[qgib]

Author Name: Giovanni Manghi (@gioman)

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I have tried with GRASS and it had the same issue.

weird, it works ok here.

I am confused thou, because I cannot understand what is wrong with that layer.
I am not trying to modify any polygons, just to merge some if they have the same sector and if they are adjacent, otherwise they should simply become "islands" with the same name.

your layer has a few geometry errors (see below), said that recently the qgis geometry support has been rewritten and a few native tools (geoprocessing tools) have now to be updated to cope with this changes.

Gap check: TopologyException: Input geom 0 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 132 and 129: TopologyException: Input geom 0 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 132 and 130: TopologyException: Input geom 0 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 132 and 131: TopologyException: Input geom 0 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 133 and 132: TopologyException: Input geom 1 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 134 and 132: TopologyException: Input geom 1 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 137 and 132: TopologyException: Input geom 1 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 138 and 132: TopologyException: Input geom 1 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 139 and 137: TopologyException: Input geom 0 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 139 and 10: TopologyException: Input geom 0 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 139 and 61: TopologyException: Input geom 0 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 139 and 131: TopologyException: Input geom 0 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 139 and 132: TopologyException: Input geom 0 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 139 and 133: TopologyException: Input geom 0 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 143 and 139: TopologyException: Input geom 1 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 143 and 132: TopologyException: Input geom 1 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556
Duplicate check between features 147 and 132: TopologyException: Input geom 1 is invalid: Self-intersection at or near point -2.4302612316084109 52.445251349342556 at -2.4302612316084109 52.445251349342556

[qgib]

Author Name: Michele Mor (@Michele100)

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Hi Giovanni,
thanks for your feedback.
If I'll have time I'll try again with GRASS.
I will try to remember to do some testing/repair for my layer next time before applying a dissolve.

[qgib]

Author Name: Alexander Bruy (@alexbruy)

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Fixed in changeset "62a571d6dd3dbdaa97f35cc6795dc064d18b7a82".

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• status_id was changed from Open to Closed

[qgib]

Author Name: Alexander Bruy (@alexbruy)

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• resolution was changed from to fixed/implemented

[qgib]

Author Name: Giovanni Manghi (@gioman)

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The "ftools" category is being removed from the tracker, changing the category of this ticket to "Processing/QGIS" to not leave the category orphaned.

[qgib]

Author Name: Jürgen Fischer (@jef-n)

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• regression was configured as 0
• easy_fix was configured as 0
• description was changed from New description, see https://issues.qgis.org/issues/13752#note-1

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Old description:

Hi.
Using QGIS 2.8.3 Wien I use Vector/Geoprocessing Tools/Dissolve to merge together polygons.
Unfortunately my new layer has an hole (missing polygons) where my original layer has polygons.
I am not able to add a screenshot within this text, so I attached it as a file.

The original layer is Units, the dissolved layer is sectors.
To dissolve the original layer I have used field PCSECTOR.

I have attached 2 zip files.

Note: using QGIS 2.12 dissolve does not work!

Thanks.
Michele
to New description, see #21779 (comment)

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Old description:

Hi.
Using QGIS 2.8.3 Wien I use Vector/Geoprocessing Tools/Dissolve to merge together polygons.
Unfortunately my new layer has an hole (missing polygons) where my original layer has polygons.
I am not able to add a screenshot within this text, so I attached it as a file.

The original layer is Units, the dissolved layer is sectors.
To dissolve the original layer I have used field PCSECTOR.

I have attached 2 zip files.

Note: using QGIS 2.12 dissolve does not work!

Thanks.
Michele

• priority_id was changed from Severe/Regression to Low

[qgib]

Author Name: Jürgen Fischer (@jef-n)

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• priority_id was changed from Low to High