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I have a map in PNG-format that looks like this: enter image description here

I have imported it into QGIS as a raster. I want to create a line layer as a network based on the blue and pink lines. Is there a way to create a line vector layer from this map without digitizing it by hand?

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    source data looks like it would be in vector format - is this data source online? - you need to georeference the png first then vectorise the colours out of the image.
    – Mapperz ♦
    Commented May 13 at 19:54
  • No the source data is not online. I just have this PNG file. How can I vectorize the colours from the grid?
    – Fabi
    Commented May 14 at 18:11
  • Probably easier to find the data as a vector layer, then split it manually based on the raster layer.
    – Bera
    Commented May 15 at 7:11

1 Answer 1

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When your png is georeferenced than you can go this way: select the png-file in the layer bar. In the main menu select Raster - Conversion - Vectorise (raster to vector). In the new window, select the png file again and start the process.

I just tried in gimp menu: tools - selection tools - select by colour, then I remove all colours except blue and violet enter image description here

with this the raster to vector conversion in QGIS works not too bad.

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  • This does not work because the blue is present in different shades. I then get a colourful mosaic
    – Fabi
    Commented May 14 at 18:13
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    Can you pre-process it in something like GIMP to convert the dark blue and pink to black, and all other colours to white?
    – Tom Brennan
    Commented May 15 at 2:25
  • Yes, that works! But how do I get the line vector layer from the raster/PNG? I tried r.thin and than r.to.vect, but that doesn't work.
    – Fabi
    Commented May 15 at 19:18

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