[Qgis-user] Geometry repair function?

[Zoltan Szecsei]

On 2012/10/30 16:13, Burgstaller Stefan wrote:
>
> @Zoltan:
>
> I will explain my problem now in detail since I think that this 
> problem could be known to a lot of users:
>
> In Germany we can order cadastre data in a proprietary format from our 
> land surveying offices. To be able to use this proprietary ASCII 
> format with common GIS software we convert the geometry objects with 
> corresponding data into point-, line- and polygon-shapes. 
> Unfortunately the source to convert contains different errors like 
> unclosed polygons (start and end coordinates are not the same), null 
> geometries, incorrect ring ordering, duplicate vertex or self 
> intersections, which are not repaired by the converting tool and 
> therefore lead to errors in the shape files. When I want to use these 
> shapefiles later for spatial functions like a select by location with 
> calculating intersections based on one of these error-shapes, the 
> results can be wrong due to the errors named above. Until now I did 
> not find any software function to solve these problems "smart" with a 
> few clicks except for the geometry repair functions provided by ArcGIS.
>
> Even the GRASS v.clean function will not solve the problems listed above.
>
> Greetings,
>
> Stefan Burgstaller
>
>
>
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Hi Stefan,
We have the same issues in South Africa with our Land Survey data.
I went a long way to cleaning the data by writing C code to check and 
log, or repair and log the source data as I converted/cleaned it for a 
client.
This was about 7 years ago.

I'm guessing when you get updates to the cadastre, you get the whole 
data set again, so you have a repeat-cleaning cycle.

In my opinion the only way to handle this is with custom code.
Take each land parcel and if it seems clean, pass it through.
Then look at the left overs to see if you can find a common thread of 
errors, program that, and re run to get a new set of leftovers.
Once that is done, you can start looking at the relative positions and 
uses (road widening or land parcel) and start figuring out the slivers 
and overlaps and other issues.

It is not as simple as I make it out to be, but it is the only way to 
protect your data from being too unreliable, and to stop you having to 
manually redo all the work again, for every update you get.
It's a big effort, but well worth it if you are the only one with a 
clean data-set :-)

I am likely to be out of office quite a lot for the next 4-6 weeks, 
otherwise I would ask you for a sample to see if I could offer better 
guidance.

Let me know how you go - and how you end up solving it.

Good luck and kind regards,
Zoltan

-- 

===========================================
Zoltan Szecsei PrGISc [PGP0031]
Geograph (Pty) Ltd.
P.O. Box 7, Muizenberg 7950, South Africa.

65 Main Road, Muizenberg 7945
Western Cape, South Africa.

34° 6'16.35"S 18°28'5.62"E

Tel: +27-21-7884897  Mobile: +27-83-6004028
Fax: +27-86-6115323     www.geograph.co.za
===========================================

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