[Qgis-user] Centre of the roads from polygons to lines

[Paulo van Breugel]

Just as a follow up on the idea by Misha. It wouldn't be too difficult 
to add a column with a unique integer value to the polygons. Then, 
follow Misha's idea, making sure that you use the newly created column 
as the raster values. After you have converted the raster back to a line 
vector layer, reconnect the attribute table using the unique integers 
you created.


On 06/03/2012 11:32 AM, Matej Mailing wrote:
> Hi,
>
> thanks, will try this one. There are many attributes and losing them
> or re-setting them is a very painful process.
>
> I was playing with different plugins and trying to use polygon
> centroids and then connecting them with lines, but had problems with
> it since it seems that gid values are not different by 1 for
> neighbouring nodes and then the point-to-line conversion is wrong.
> Perhaps some sort of a functionality to correctly connect polygon
> centroids with a line would be most welcome and I am sure it could
> also be used in some other cases.
>
> Thanks,
> Matej
>
> 2012/6/3 Micha Silver<micha at arava.co.il>:
>> On 02/06/2012 11:02, Matej Mailing wrote:
>>
>> Hi,
>>
>> I have a layer that contains roads in polygons and I need to get
>> central lines of the polygons. When I convert directly from polygons
>> to lines, I get two lines for each part of the road representing road
>> "boundaries" which isn't good.
>>
>>
>> There's a procedure using GRASS that you might choose. It involves
>> converting the polygons to a GRASS raster, then running the GRASS module
>> r.thin, and then converting back to a vector.
>> Here's an old thread that mentions the method:
>> http://www.mail-archive.com/grass-user@lists.osgeo.org/msg01015.html
>>
>> Two difficulties:
>> * you have to set the resolution fairly high in order to keep the road's
>> shapes, but then the raster might be too large to work with on a large area
>> * and you loose the attribute table on the way...
>>
>> What is the best way to achieve it?
>>
>> Thanks,
>> Matej
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>>
>>
>> --
>> Micha Silver
>> 052-3665918
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