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I used QGIS to georeference a raster file of a custom US map. The map has features which are not strictly defined by state borders. I would like to digitize the map by creating contiguous borders for each feature. I tried that a few different way, but none of them really worked:

  • traced the borders with polylines, and digitizing with the Polygonizer plugin. The plugin kept crashing.
  • used Conversion > Raster > Vectorize (Raster to Vector). That one worked, but the result was many overlapping polygons for each feature, each with slightly different borders.

I have somewhere between 50-100 different raster images to process, which is why any automated solution is highly preferred. I'm new to GIS software, but could easily pre-process the images if needed, either manually or with a custom script (using Python/PIL, ImageMagick, Photoshop batch processing or similar tools).

Thanks for any pointers!

Original Original image

Georeferenced image Screenshot of georeferenced image

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3 Answers

What you could do is:

  1. In the original image, fill each area with a unique color, e.g. with Gimp
  2. Georeference
  3. Run Raster to Vector
  4. Clean up the polygon geometries manually
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That sounds like a good way to approach it. Would you suggest to get rid of the boundaries once I colored each polygon in the raster? –  poezn Aug 20 '12 at 21:13
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Hey I know you prob have the best answer above with the fill option. I was looking to identify individual Polygons on a raster image, cut them out and place them as a shapefile layer in qgis and this was the answer i got....may be of use in future:

Identify Polygons on raster image

if not dont vote down!! it may not work as the situation is different and all boundaries are likely to have the same properties.

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I think your problems might have started with the CRS (the "projection") you selected for georeferencing the raster. It looks as if you have taken a projected raster map image of the contiguous United States and georeferenced it using lat/lon coordinates, which has produced a very distorted and "unnatural" looking result.

For mapping the whole of the Lower-48 the USGS (and the Census Bureau too, I think) use an Albers Equal Area projection. Your original raster map looks as if it might use an Albers projection, so it would make sense to try georeferencing it to this CRS (in QGIS this is EPSG:102003). You will need to use EPSG:102003 coordinates to do this, come back if you have problems obtaining these.

If you georeference your raster map to Albers I think that your problems will go away, including the polygonizer one and your map will look better too.

Regards, Nick.

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Thanks nhopton! I realized that the projection was less than optimal. The goal for me was really to get started with the georeferencing process with the goal to change the projection later. I don't quite see how a different projection would help me with the vectorization, though. My problems still persist that that boundaries are 2-4 pixels wide, and so the vectorization creates multiple versions of the same feature. –  poezn Aug 20 '12 at 21:11
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