I have a raster map which I have extracted from a government report providing flood extent return frequencies. I want to 'snatch' these flood extent polygons (i.e. convert to vector) and I was wondering if there is a simple way to do this in QGIS. Simply digitising these layers by tracing new polygons would be too time consuming and potentially result in human error.

Is there a tool or methodology someone could recommend to me which would recognise these polygons in raster format (there are clearly outlined and coloured distinctly differently to the background) so I can convert them to vector?

Any help with this is very much appreciated.



It can be done using the QGIS GRASS Plugin..you can use r.to.vect for your task..I found a demo video of this in youtube..It is an excellent demo..check this link

here is a snap of the tool

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GDALTools plugin offers direct Vectorization without having to import the data into GRASS first. You find it in Raster menu once it's installed and activated.

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  • i tried this using a simple map..for a 50 square feet area map, it created more than 1 million polygons..it creates polygons for each pixels..maybe my usage is not correct? – vinayan Mar 30 '12 at 17:26
  • Do all cells within the areas of interest have the same pixel value? How did you extract the map from the report? Which format is the raster in? – underdark Mar 30 '12 at 19:55

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