[Qgis-user] Repairing Topology Errors

Mon Feb 8 13:45:48 PST 2016

Hi,

Am 08.02.2016, 20:26 Uhr, schrieb Springfield Harrison  
<stellargps at gmail.com>:

> 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.
In the list of things you do while digitizing, I miss the option to tick  
"Avoid intersections"-checkbox in the snapping option for your polygons  
layer. Actually, I never snap to existing vertices (maybe only the first  
and the last of adjacent polygons), but simply click into the adjacent  
polygons and then out again. Ticking "Avoid intersections" will  
automatically create the common border of the new polygon with the new  
one. So there is generally no need for any autotrace or carefully  
digitizing on existing vertices

Maybe you just forgot to mention this, but having many errors looks to me  
that this option might be missing in your workflow.

>
>
>        Any suggesting on repairing topology errors easily?

Cleaning up topology errors is best done with GRASS v.clean through the  
processing toolbox.
I normally use the bpol option, but more important is to use the advanced  
options there and setting the thresholds for snapping and the minimum area  
for resulting polygons. You might have to experiment with those settings  
and check the results.
If you still have slivers cause the snapping can't connect enough existing  
vertices, try "Densify geometries given an interval" to add more vertices  
before the cleaning. You can remove those extra vertices later again with  
"Simplify geometries" from the tool box.
That's my usual workflow, maybe other have better tools.
I never bother to use the topology checker, am simply too lazy for that  
and doing this stuff manually.

>
>
>                 Thank you, Cheers . . . . . . . . Spring Harrison
>
Good luck
Bernd

>
>
> 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 >:
>> 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 ÂÂ Â Â Â Â Â Â Â Here are some product details:Â QGIS  
>> versionÂ 2.10.1-PisaÂ QGIS code revisionÂ 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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-- 
Bernd Vogelgesang
Siedlerstraße 2
91083 Baiersdorf/Igelsdorf
Tel: 09133-825374
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