I am trying to find road intersections within each of my memory Polyline layers (which are named road_layer_i for i in (0-99)) so this is what I tried:

import processing

layers = self.iface.mapCanvas().layers()

for layer in layers:
   for i in range(0,100):
      if layer.name()=='road_layer_%s' % (i) :
        processing.runalg('qgis:lineintersections', layer, layer, 'id', 'id', 'C:/Phoenix/intersection_%s.shp' % (i))
        IntersectionLayer=QgsVectorLayer('C:/Phoenix/intersection_%s.shp' % (i) , 'intersection_%s' % (i), "ogr")

I don't get any error message but each layer created contains points that are the road intersections for the layer 'road_layer_0'.

I really don't understand why it does that so I would be grateful if someone could tell me what I did wrong.

Also, whenever I simply try

import processing

layers = self.iface.mapCanvas().layers()

for layer in layers:
  if layer.name()=='road_layer_33' :
    processing.runalg('qgis:lineintersections', layer, layer, 'id', 'id', 'C:/Phoenix/intersection_33.shp')
    IntersectionLayer=QgsVectorLayer('C:/Phoenix/intersection_33.shp' , 'intersection', "ogr")    

I still get the intersections for the road_layer_0...

After answer by @gcarrillo it is still not working. I indeed obtain 100 layers labbeled interesection_i (i in 0-99) but they all contain only the intersections for road_layer_0. Here is the code I used to create my road layers :

from fTools.fTools import fToolsPlugin
import shapely
from qgis.core import QgsRasterLayer, QgsCoordinateReferenceSystem, QgsGeometry

RoadLayer=QgsVectorLayer("C:/Phoenix/Project/Phoenix/Phoenix_roads.shp" ,    "Phoenix_roads", "ogr")
RoadLayer.setCrs( QgsCoordinateReferenceSystem(3857, QgsCoordinateReferenceSystem.EpsgCrsId) )
#Get the data from this layer
provider = RoadLayer.dataProvider()
fields = RoadLayer.pendingFields()
field_names = [field.name() for field in fields]

# create list of points used to create rectangles 
for i in range(0,100):
#    create new layer in which we will put the roads that are inside the rectangle
   layer =  QgsVectorLayer('LineString', 'road_layer_%s' %(i) , "memory")
   pr = layer.dataProvider()
   attrib_layer=input_layer_attrib_names = RoadLayer.dataProvider().fields()
   oldattributeList = RoadLayer.dataProvider().fields().toList()
#Generate attributes in destination layer
     for attrib in oldattributeList:
        if layer.fieldNameIndex(attrib.name())==-1:
  #    add empty attribute fields to new layer
  #create points left-bottom and right-top of rectangle we want to select
     pointA = QgsPoint(x[0],x[1])
     pointB = QgsPoint(y[0],y[1])
   #   create rectangle
   #   create request
     request = QgsFeatureRequest().setFilterRect(Rect)
     for f in RoadLayer.getFeatures(request):
    #        add geometry from road to new road
         pt = QgsFeature()
    #       add attributes
         atr = dict(zip(field_names, f.attributes()))
     #        add new feature to layer
    #        add layer to map canva

Is there anything wrong with this method that makes the use of processing.runalg('qgis:lineintersections', ... ) impossible to iterate over the layers thus created ?

  • It's nothing wrong with such algorithm, I've tested it with 11 layers on my QGIS and I got the expected results: different intersection point layers for input line layers. Just a question: Are you sure you have 100 layers with different roads (even if their name is different, they could still contain the same lines)? Please double check your input data, because this doesn't seem to be a programming problem but one related to your data. – Germán Carrillo Feb 11 '15 at 1:18
  • My layers have indeed different roads, which is why i find it really weird. maybe the problem is that they are memory layers. For instance i tried saving a few of them as regular vector layers and processing runalg worked on these. My only problem is that i do not know how to convert memory layers into regular layers via python (or maybe just change my code so that i directly create non-memory road_layers). – JDO Feb 11 '15 at 1:21
  • And it case I can't make it work on memory layers, can you tell me how to convert them into regular layers ? – JDO Feb 11 '15 at 1:27
  • Yeah, I think that's better. So, have a look at my answer to Export several files at the same time in QGIS. That would export all your layers as Shapefiles to a folder. – Germán Carrillo Feb 11 '15 at 1:49
  • 1
    using memory layers (which do have an 'id' field i think), it does run ok when using the processing toolbox but not when running it in the python console. Anyway I did as you suggested, exported all my files first and then run the algorithm and it worked just fine ! Thanks a lot ! – JDO Feb 11 '15 at 3:35

I assume your road_layer_i (for i in [0-99]) layers are not the same layer, that is, they do not have exactly the same lines.

Having said that, I've modified your code snippet a bit, making these changes:

  • Use QgsMapLayerRegistry.instance().mapLayers().values() instead of self.iface.mapCanvas().layers(). This ensures you'll iterate on all layers and not only on those that are visible.

  • Instead of checking matching names, extract the i from the layer name.

  • Load all resulting intersection layers at once instead of one by one.

This is the resulting code:

import processing
layers = QgsMapLayerRegistry.instance().mapLayers().values()

for layer in layers:
    i = layer.name().split("_")[-1]
    processing.runalg('qgis:lineintersections', layer, layer, 'id', 'id', 'C:/Phoenix/intersection_%s.shp' % (i))
    outputLayers.append(QgsVectorLayer('C:/Phoenix/intersection_%s.shp' % (i) , 'intersection_%s' % (i), "ogr"))

QgsMapLayerRegistry.instance().addMapLayers( outputLayers )

I've checked it on GNU/Linux, QGIS 2.6.1.

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