I want to extract certain values of a raster image in order to create a new raster which then should only contain the extracted/chosen values.
How to do this in QGIS?
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Load the raster and start the raster calculator. Set an output filename then in the "Raster calculator expression" text box type:
filename@1 = 3
Where "filename" is the name shown under "Raster bands". Then press OK. That's it!
QGIS has a Raster Calculator that can calculate specific raster bands using mathematical expressions and save that to specific GDAL enabled formats, such as GeoTIFF, Erdas Imagine, etc.
There are a number of additional QGIS plugins as well.
see http://plugins.qgis.org/plugins/tags/raster/ QGIS can be built with GRASS which has raster functionality http://grass.osgeo.org/programming6/gisrasterlib.html
You didn't ask about this, but you might want to consider other remote sensing software such as Optics [FOSS]-- opticks.org
And these industry standard Products of course: Erdas Imagine geospatial.intergraph.com and the exceptional ESRI Raster capabilities. www.Esri.com
Gdal calc would be another option use
This was a similar question. I was looking to take certain parts of the raster out (in my case buildings) and create polygons...but you create a raster first. or create the polygons and then convert to raster.