Does anyone know how to calculate the area in km² for each altitudinal band (between 100 and 200, 200 and 300...) based on a DEM? I added the contour lines with a distance of 100 metres but could not find a way to modify them in order to allow area calculation. I am working with qgis 1.8.0. DEM is taken from ASTER.
My initial answer was wrong so im providing a new one:
As wuber said the best way to calculate the area to reclassify the parts of your dem you want measure, polygonize (raster to vector) then add the $area attribute. Here's a step-by-step example.
For the purpose of this example im using a DEM which can be downloaded from here.