[Qgis-user] Fix Geometries

Fri May 24 00:20:43 PDT 2019

Hi,

We published a document in early 2018 on geometry fixes.
This document should be revised, but may provide some elements.
(For example, geometry checker is now more stable under QGIS 3)

It is open for comments...
We have in our projects to produce a more user-friendly version

see : 
http://geoinformations.metier.e2.rie.gouv.fr/spip.php?page=article&id_article=3522&id_rubrique=712
see the document at the bottom of the page

Regards,
Alain

FERRATON Alain
Ministère de la Transition écologique et solidaire (MTES)
SG/SPSSI/CPII/DOO
Chef du groupe Expertise Technique
02.40.12.84.08

Le 23/05/2019 à 13:07, > Richard McDonnell (par Internet, dépôt 
qgis-user-bounces at lists.osgeo.org) a écrit :
> Hi Finbar,
> One other solution would be to use a double buffer, as in a positive buffer something like 0.001, followed by a negative buffer of -0.001.
> The issue you are facing is common, as most flood polygons are derived from raster's and you can get a lot of self-intersections. The double buffer is in my experience the handiest method of solving the issue. Also by using such a small buffer, even a single buffer, will make little difference, to a house being in or out of a flood zone, when taking into account the, resolutions of the polygon datasets.
> Hope this helps,
>
> Regards,
>
> Richard.
>
> -- 
>
>
> Richard McDonnell
> GIS Specialist | FRM Data Management
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> Email:  richard.mcdonnell at opw.ie
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> -----Original Message-----
> From: Qgis-user <qgis-user-bounces at lists.osgeo.org> On Behalf Of GILLEN Finbar
> Sent: 22 May 2019 11:40
> To: qgis-user at lists.osgeo.org
> Subject: Re: [Qgis-user] Fix Geometries
>
> Thanks for all the responses on this... I am going to try the Check Validity plugin ('QGIS' method) as a starter within QGIS 3.2. I am using a fairly detailed flood polygon so accuracy is key. I will let you know how I get on.
>
> Thanks again,
> Finbar
>
> -----Original Message-----
> From: Qgis-user <qgis-user-bounces at lists.osgeo.org> On Behalf Of Nicolas Cadieux
> Sent: 21 May 2019 23:02
> To: qgis-user at lists.osgeo.org
> Subject: Re: [Qgis-user] Fix Geometries
>
> Hi,
>
> The Geometry Checker plugin will add a the 'Check Geometries' option in the vector menu.  It is very unstable with the current version of QGIS. If you get it to run, it should solve most problems.  I get it to run when I only select one test at a time.
>
> The most predictable tool is the vector/geometry tools/Check validity using the 'QGIS' Method (not GEOS).  That should fix your file.  There is a 'remove duplicate vertices' and 'delete duplicate geometries' in processing.  Your problem could also be caused by a module not working with multipart objects.  Go to vector/Geometry tools/Multiparts to singleparts.
>
> If your lines don't touch,   try the 'snap Geometries to layer ' in processing.
>
> Nicolas
>
> On 2019-05-21 2:54 a.m., Mithoefer wrote:
>> Hi Finnbar,
>>
>> What plugin are you talking about and what QGIS version do you use?
>> You have a number of tools to check geometries. Try "Topology checker"
>> or "Geometry checker" to see where the problems are (invalid
>> geometries). Geometry checker is also able to fix some issues. Maybe
>> thats the one you use.
>>
>> Anotehr option is GRASS GIS. Have you ever worked wit GRASS GIS? You
>> can create a GRASS database and import your polygons. GRASS GIS uses a
>> topolgical data model and is quite powerful in establishing a desent
>> data model with snapping options and clean functions. You can also try
>> GRASS tools from the processing framework like build polygons or clean.
>>
>> Regards
>> Klaus
>>
>> Am 20.05.19 um 11:31 schrieb GILLEN Finbar:
>>> Hi All,
>>>
>>> I have a few polygon layers where anytime I go to use them I have to
>>> use the fix geometries plugin so that invalid geometries are fixed.
>>>
>>> When I save the output from the fixed geometries plugin and add it in
>>> a new project the same issues appears where I can’t use it accurately
>>> to do a Select or Join by Location because some of the geometries are
>>> still seen as invalid even though they have been ‘fixed’.
>>>
>>> Is there anyway solution to this or do I have to keep using the fix
>>> geometries plugin anytime I use the polygons for analysis?
>>>
>>> Regards
>>>
>>> Finbar
>>>
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