So I've been trying to convert a georeferenced Tif raster file into a vector shp file using QGIS. It's seems to complete this but it ends up giving me a feature class that has over 500,000 features, almost like it's creating a row per pixel. So with that many features it's very hard to work with and when I do finally get it to Classify like into 8 classes of natural breaks, it doesn't look like the original tif file. My end goal is to only have a few (less than 20) polygons based off the raster not thousands. Could it be that the Tif file I'm using is already pre-processed or something?
It sounds as if your raster needs to be classified, which you can do using the experimental Sextante toolbox.
Each pixel in a one-band raster has a value attached to it and using GRASS v.reclass from the Sextante toolbox allows values to be grouped into classes. Say for example I have a raster with values of between 0 and 1400; using v.reclass I could group the values into seven classes, using this 'rules' text file:
0 thru 200 = 1 201 thru 400 = 2 401 thru 600 = 3 601 thru 800 = 4 801 thru 1000 = 5 1001 thru 1200 = 6 1201 thru 1400 = 7
The output would be a new raster having a value of 1 for all of the values of between 0 and 200 in the original raster, of 2 for all of the values between 201 and 400 in the original raster, and so on.
Once classified, the raster could then be polygonised.