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I am using Quantum GIS 1.8.0. I have a base map sourced from OSGB Opendata VectorMap District (vector format), and that displays fine. I also have created a spreadsheet showing journey times to a chosen destination, with the grid references of 4623 postcodes within 10km of that destination. The first few lines of this spreadsheet are:
from easting northing time
DH1 1AB 430317 543630 527
DH1 1AD 430239 543638 534
DH1 1AE 430220 543685 545
DH1 1AF 430340 544013 555
DH1 1AG 430323 543876 556
I have imported this as a delimited text layer (is that the right terminology) and my map shows the locations of all these postcodes.
What I want to do is to plot contour lines showing the areas that are within 5 / 10 /15 / 20 minutes of the destination and shade them. (Is this called an isochron?) When I try to use the contour plugin to do this, it goes to 99% cpu and the progress bar does not move off zero. I should perhaps add that postcodes are packed quite densely in settlements and quite far apart in open countryside.
There must be a better way to do this, but as a newby to this software I need some advice please.

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    try to save your layer (the imported delimited text layer) as shapefile (right-click -> save-as). Open the shp in qgis and try the contour plugin again
    – Curlew
    Sep 6, 2013 at 10:37
  • Thanks @Curlew. However, the layer was already a 127k shapefile. I did try saving it under a different name and trying again, but with the same result.
    – Roger
    Sep 6, 2013 at 13:43

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I don't have the contour plugin in front of me right now but I think I remember that it uses a raster layer as input. You would therefore first create a raster by interpolating the point values and then create the contours.

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  • Thanks for the suggestion. However... the input section of the Contour plugin asks for the Vector layer and Data field, so your recollection that it uses a raster layer is not correct.
    – Roger
    Sep 7, 2013 at 11:19

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