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I was wondering if there is a way to create centre lines from polygons in QGIS? I have a polygon layer, but ideally need the centrelines as a separate file.

Thanks, Alan

Example

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Given that polygons are non-linear and don't neccessarily carry any particular vector, they don't have centre lines to extract, nor would be an easy way to 'extract' them programmatically without seeing a sample of the data. How complex is your dataset? Where did you get it? –  nagytech Sep 20 '12 at 10:04
    
I've just uploaded a picture of the polygons –  Alan Carr Sep 20 '12 at 10:39
    
Where, in the example image, would the 'centrelines' fall? Could you update the image showing the desired lines in a different colour, perhaps? –  Simbamangu Sep 20 '12 at 10:55
    
Just directly down the middle of all polygons. The final outcome we need would be to just find the length of the whole thing (assuming the polygons were lines) so it's not even essential to get centrelines. –  Alan Carr Sep 20 '12 at 11:09

3 Answers 3

there is a good tool with the name of ET GeoWizards 10.2 for Esri products, but i dont know whether it is a good solution or not...

you can get some info from here

centerLines

beside this in grass you can check out r.thin function for basic use..

r.thin - Thins non-zero cells that denote linear features in a raster map layer.

rthin i hope it helps you...

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this is a ready to use tool, thanks –  geogeek Sep 20 '12 at 11:49

you can try This algorithme:

get vertices from the polygons(poly_vertex).
-make voronoi using (poly_vertex) as input the result could be called(poly_voronoi).
-get lines from (poly_voronoi) => (poly_voronoi_lines).
-get the originale polygon contour (polygon_lines).
-remove (poly_voronoi_lines) touching  (polygon_lines) the result is supposed to be the center lines , but you may need further clean up.

i don't know how it could be implemented on Qgis

i wish it could help

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What other programs can you do this on. I can get access to MapInfo and Arc if neccessary –  Alan Carr Sep 20 '12 at 11:35
    
this algorithme, will work surely in Arcgis and it could work too in Qgis using Voronoi polygons gis.stackexchange.com/questions/6971/thiessen-polygons-in-qgis –  geogeek Sep 20 '12 at 11:39

just find the length of the whole thing

How exactly should be the result? An approximation is perimeter divided by 2.

To get a better result: You can manually measure some polygons.Then determine the difference between the measurement and perimeter divided by 2. Apply this correctionfactor to the whole datset.

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