Re: [Qgis-user] Repairing Topology Errors

Mon, 08 Feb 2016 23:44:57 -0800 

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 Cerdán 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 "Configuration --> Snapping options --> Change the
Self-assembling mode to "advanced" and be sure that "avoid
intersections" is marked in your target layer only.

Cheers,

Carlos Cerdán

2016-02-02 20:57 GMT-05:00 Springfield Harrison <[email protected]
<mailto:[email protected]> >:

    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. 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. 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. No error
    messages were created. 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-Pisa QGIS code revision 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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