I'm trying to create a an area that can be reached within a certain amount of time/distance from a single (or multiple) points. To do this i'm using the GRASS tool v.net.iso (via the GUI). I've succesfully made a Grass routable network with two layers (arcs and nodes) via the v.net tool.

The trouble starts when I want to use the v.net.iso tool. I select the input arcs and input nodes to be my created network. But than I have to input "cats (select from the map or using their id)" When I hover mouseover, it sais the following: "Categories of centres (points on nodes) to which net will be allocated. Layer for this categories is givan by nlayer option.

Do I just type the coordinates from one of the nodes? e.g. "xcoordinate|ycoordinate". Because if I do this, I get the following error: "Not enough centres for selected nlayer. Nothing will be allocated."

And can I let the cost isolines be dependant on an attribute? Or can I only manually enter the cost isoline in mapunits?

  • How did you create the specific layer with the point you wanted as the centre of the isochrone? You specify v.net, but which one? Is there an option within the v.net.iso GUI? I've managed to identify the CAT of the individual nodes created from v.net.nodes, although how do I create a layer with just one node in it? Cheers, any help would be greatly appreciated!
    – Adam Bray
    Sep 15, 2016 at 14:52
  • 1
    Hi Adam, You need a seperate point layer with the point you want as centre. You can add this point layer to the network layer by the v.net.connect module. In QGIS 2.14.3 (and further) the processing toolbox includes discriptions about what each module does. Does this help?
    – LMB
    Sep 17, 2016 at 9:09

1 Answer 1


I managed to get it working. Let me elaborate. I had to add an aditional nlayer to the network. I did this with the v.net tool. Thus in the end i had 3 layers in the network. 1. the lines being the network itself. 2. the nodes at the end of each line segement. 3. the point i wanted to use as center of the isochrone. (you can check this via the browser by opening the network database) Then with the identify feature I was able to identify the 'cat' of the node i wanted to use as center. This 'cat' (in this case 1, which is guess means it is the first node layer other than the nodes needed for the calculations) i used as cat input in the v.net.iso gui. This gave me the results i wanted.

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