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The code for shortest path analysis with PyQGIS specifies start and end nodes explicitly:

pStart = QgsPoint( -1.37144, 0.543836 )

pStop = QgsPoint( -1.1027, 0.699986 )

http://www.qgis.org/pyqgis-cookbook/network-analysis.html

Does anyone know whether it's possible to replace these statements with sets of all nodes, so that it's possible to get all-paired shortest path?

Thanks

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  • Couldn't you just loop over all the node pairs and call the functions? Commented Mar 29, 2013 at 11:00
  • @NathanW That's what I wanted to do, but my lack of programming skills is holding me back: how should I declare a set of all points that are in my network? Commented Mar 29, 2013 at 13:32
  • @NathanW Because there are 900 nodes, so I'm looking for a way to programmatically declare all the nodes Commented Mar 29, 2013 at 18:16
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    As Nathan pointed out this is just about applying a loop over a point layer. Also have a look at the QgsDistanceArea() object, which is quite straight forward. qgis.org/pyqgis-cookbook/measure.html .Try it yourself first and report where you have problems Commented Mar 30, 2013 at 15:19

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OK, so this is what I came up with; it might be useful for novices like me out there. I know it's not perfect at all, so any edits are welcomed!

Given: I've a road network and a point layer which has x, y and TAZ( i.e. Traffic Analysis Zone id) with indexes of 191,192 and 1, respectively.

Desired: a distance matrix for all the points. for example, I'm interested in finding the distance from points with TAZ's of 125 to 135 to all the other points.

 x=[] y=[] taz=[] dict={} pntLayer=qgis.utils.iface.mapCanvas().currentLayer() //select point layer in TOC feat=QgsFeature() provider=pntLayer.dataProvider() provider.select([1,191,192],QgsRectangle(), True, False) while provider.nextFeature(feat): // fetch attributes, returns qgis objects ... m=feat.attributeMap() ... x.append(m[191]) ... y.append(m[192]) ... taz.append(m[1]) ... // I want a dictionary with points as keys and TAZ as values xp=[] yp=[] tazp=[] for i in taz: ... tazp.append( str(i.toString())) for i in x: ... a,v=i.toDouble() ... xp.append(a) ... for i in y: ... a,v=i.toDouble() ... yp.append(a) ... qPoints=[] for i in xrange(900): // point layer has 900 entries ... qPnt = QgsGeometry.fromPoint(QgsPoint(xp[i],yp[i])) ... qPoints.append(qPnt) // now I have reconstructed the points from x and y ... net=qgis.utils.iface.activeLayer() // select network layer in TOC properter=QgsDistanceArcProperter() director = QgsLineVectorLayerDirector( net, -1, '', '', '', 3 ) director.addProperter(properter) crs=QgsCoordinateReferenceSystem(26986, QgsCoordinateReferenceSystem.PostgisCrsId) builder=QgsGraphBuilder(crs) qp=[] for i in qPoints: ... x=i.asPoint() ... qp.append(x) ... d={} for i in range(900): ... d[qp[i]] = tazp[i] // d: dictionary with points as keys with corresponding TAZ tiedPoints=director.makeGraph(builder, d.keys()) graph=builder.graph() //tiedPoints are now different points from d.keys() but the order is preserved. // so I'll create a new dictionary and connect new points to their corresponding TAZ values. dict={} i=0 for j in d.keys(): ... dict[tiedPoints[i]] = d[j] ... i=i+1 ... t={} // dictionary of graph id and TAZ for j in xrange(900): id=graph.findVertex(dict.keys()[j]) ... t[id] = dict[ dict.keys()[j]] costMat={} //cost matrix with graph id as keys for id in t.keys(): ... (tree, costs) = QgsGraphAnalyzer.dijkstra(graph, id,0 ) ... c=[] ... for i in t.keys(): ... c.append(costs[i]) ... costMat[id]=c ...
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