Geographic Information Systems Stack Exchange is a question and answer site for cartographers, geographers and GIS professionals. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.


share|improve this question

It can be done using the QGIS GRASS can use 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

enter image description here

share|improve this answer

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.

enter image description here

share|improve this answer
i tried this using a simple map..for a 50 square feet area map, it created more than 1 million 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

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.