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I have a .shp file, and I need to get the dual graph from this file (that is to say, each polygon centroid should become a vertex, and two vertices are linked if their associated polygons are adjacent).

I managed to get the centroids from the polygons of my file.

How can I add lines between the vertices that should indeed be connected?

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  • Are you looking for an out of the box tool in QGIS or a script? – whyzar Apr 5 '18 at 13:41
  • At this point, every solution is a good solution to me. – Alexandre Apr 5 '18 at 14:10
  • I can totally understand that however, an out of the box tool in QGIS may not be available. If you want a script, you will have to at the very least show an attempt of creating a script that can be helped with. – whyzar Apr 5 '18 at 14:15
  • What form do you want the lines? As a set of line features with little link back to the original polygons? – Spacedman Apr 5 '18 at 14:52
  • Related to Can GRASS compute the line graph representation of an input graph? – underdark♦ Apr 5 '18 at 17:07
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You can use Python only (versions 2.7.x and 3.x) without QGIS:

enter image description here

1) With Fiona, shapely and itertools

import fiona
from shapely.geometry import MuliPolygon, LineString, mapping
Multi = MultiPolygon([shape(poly['geometry']) for poly in fiona.open("polygons.shp")])
# creation of the dual graph shapefile
import itertools
# schema of the dual graph shapefile
schema = {'geometry': 'LineString','properties': {'test': 'int'}}
with fiona.open('dual_graph.shp','w','ESRI Shapefile', schema) as e:
   for poly1,poly2 in  itertools.combinations(Multi, 2):
       if poly1.touches(poly2):
           e.write({'geometry':mapping(LineString([poly1.centroid, poly2.centroid])), 'properties':{'test':1}})

enter image description here

2) with GeoPandas,shapely and itertools

import geopandas as gpd 
polygon = gpd.GeoDataFrame.from_file("polygons.shp")
geoms = polygon['geometry'].tolist()
dual = gpd.GeoDataFrame(gpd.GeoSeries([LineString([poly1.centroid, poly2.centroid]) for poly1,poly2 in  itertools.combinations(geoms, 2) if poly1.touches(poly2)]),columns=['geometry'])
dual.to_file("dual_graph2.shp")

3) I suppose you can do the same thing with PyQgis and itertools (Python 2.7 for 2.x versions and 3.x for 3.x versions)

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  • The first code works fine! I might just add that it should be "as e:" instead of "as e::", and that in my case (I didn't want edges between polygons sharing only one point) I added the condition " (poly1.intersection(poly2)).geom_type!="Point" " to the if loop. – Alexandre Apr 6 '18 at 8:31

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