[Qgis-user] Repairing Topology Errors

Tue Feb 9 00:14:45 PST 2016

Those are more issues with neighbouring polygons - you can also get (depending on source data) unclosed polygons & self overlaps - butterfly or figure 8 polygons - which are also issues in polygon topology.
Unclosed polygons are formed when non-touching linestrings are used to build a topology from, & the topology snapping tolerance is smaller than the gap between the line vertices.
A self intersection is usually a digitising error. Like you , I use an external program to create topologies - you suggested GRASS, in my case I use Postgis, & once the topology structure is built from the input linestrings, you can extract/derive the resulting polygons.

I have not had this problem in QGIS when digitising with snapping is on - provided it is on for points & vertices, all the relevant layers & the tolerance is set appropriately.
Postgis has tools to identify the locations of errors, zooming in to that location with QGIS with snapping on has let me fix them manually. I usually create the underlying linestrings, then the topology, then the polygons. Any errors are addressed in the original linestrings, then the derived data recreated. Repeat until it is acceptable. 

I have a minor issue with some auto fix tools, as they typically involve moving one point (or vertex) to the same location as another. However - you do not always have a say in which point of the two gets moved - & it can be the wrong one.  

Brent Wood

      From: Bernhard Ströbl <bernhard.stroebl at jena.de>
 To: qgis-user at lists.osgeo.org 
 Sent: Tuesday, February 9, 2016 8:44 PM
 Subject: Re: [Qgis-user] Repairing Topology Errors

Hi,

when looking at topology errors I can imagine three types
1) borders are not identical, i.e. you missed one vertex during digitizing
2) overlaps
3) gaps

ad 1) in my experience it is easy to miss a vertex when digitizing along 
an existing edge, the auto-trace plugin may help in this respect and 
also "avoid intersections" (although you seem to have had problems with 
it). Snapping must be activated, of course. You cannot snap without it 
because you never hit the exact same point twice.
So for your edit layer you should snap to vertex with a reasonable 
snapping distance and "avoid intersections". Thus you should be set to 
not miss any vertices.
fix: only manually AFAIK
2) have "avoid intersections" active
fix: I use GRASS with its topological data model to identify all 
averlaps and use Processing's "eliminate sliver polygons" to get rid of them
3) no way to avoid them because IMHO they could be intended
fix: same as 2)

Bernhard

Am 08.02.2016 um 20:26 schrieb Springfield Harrison:
> Hello Group,
>
> I have been manually creating a few simple polygons, always intending to
> have no gaps, slivers or overlaps.  To do this I use snapping and
> occasionally the Autotrace feature.
>
> However, when I run the topology checker, it often flags a variety of
> errors although I am careful to create polygons by only snapping to
> existing vertices of adjacent polygons.  It does a great job of finding
> the errors, but I can find no instructions on methodology for repairing
> them.
>
> Any suggesting on repairing topology errors easily?
>
> Thank you, Cheers . . . . . . . . Spring Harrison
>
>
>
> At 03-02-2016 07:51 Wednesday, Carlos CerdÃ¡n wrote:
>> Hi Springfield:
>>
>> I've had similar trouble some time ago, as a QGIS beginner user.
>> Previous given advices are for not too-beginner users, but I think my
>> experience can be useful for the list (or even for you?):
>>
>> That time my error was my Snapping options: I had selected two or
>> three layers and it was selected the "avoid intersections" option,
>> then, when I added new digitized polygons, they were splitted or
>> deleted (as you tell) because new polygons were intersecting marked
>> layers in snapping options. It fixed when new digitizing shape was the
>> one marked (of course to avoid intersections between her elements) .
>>
>> Yes it was something simple, my foul. Is it not same trouble?. If so,
>> Check it in "ConfigurationÂ --> Snapping options --> Change the
>> Self-assembling mode to "advanced" and be sure that "avoid
>> intersections" is marked in your target layer only.
>>
>> Cheers,
>>
>> Carlos CerdÃ¡n
>>
>> 2016-02-02 20:57 GMT-05:00 Springfield Harrison <stellargps at gmail.com
>> <mailto:stellargps at gmail.com> >:
>>
>>    Hello Group,
>>
>>    Â Â Â Â Â Â Â Â Am using QGIS 2.10.1 and have done so without
>>    significant problems for many months.Â I am attempting to create
>>    simple polygons in a shapefile by direct vertex entry or copying
>>    polygons from another shapefile layer and pasting them into the
>>    active layer.
>>
>>    Â Â Â Â Â Â Â Â In almost every case, these features disappear.Â
>>    There is an entity entry in the attribute table but there are no
>>    polygons created.Â In one case a polygon was created but it was
>>    not the shape that I digitized; no more polygons could be created
>>    although the process for doing so appears to be working.Â No error
>>    messages were created.Â The CRS for the project and the shapefile
>>    are the same.
>>
>>    Â Â Â Â Â Â Â Â The shapefile can be opened in another GIS but has
>>    no content.
>>    Â Â Â Â Â Â Â Â
>>    Â Â Â Â Â Â Â Â Here are some product details:Â QGIS versionÂ
>>    2.10.1-PisaÂ QGIS code revisionÂ d20c5b7
>>    <https://github.com/qgis/QGIS/commit/d20c5b7>
>>
>>    Â Â Â Â Â Â Â Â Thanks very much for any help you can render,
>>    Cheers . . . . . . . . Springfield Harrison
>>
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