How to generate Non-smooth/crude Contour Lines in QGIS?

Question

I have an SRTM DEM and I want contour lines generated for specific elevations. This can easily be done using r.contour.level/step etc. but the contours generated are smooth and cover the cells only partially (red lines in image).

Are there any methods by which I could generate a contour line that encloses the cell boundaries above a desired elevation.

Image description: Red line is the 3950m contour generated by QGIS. The yellow line is one I need: a contour enclosing all cells >= 3950m).

EDIT: The goal is to extract the surface area of elevation ranges (50-100,100-150 etc.) in my raster. I have a surface area raster, and the next step is to calculate zonal statistics of the raster based on a defined polygon area of interest as my contours are occur every 50m, they seemed ideal for the mask. BUT Using a smooth contour vector gives me incomplete zonal statistics for every elevation polygon. This is could be because the 3950 contour polygon does not "completely" enclose pixels >= 3950. In the example below, the the zonal stats only gave me data for 5 pixels. The correct data should include 12 pixels (which I manually checked to be >= 3950m) as enclosed by the yellow line.

Answer 1

Hmmm, interesting problem! Although I'm not quite sure why you'd need to do it.

I would approach it by first converting the raster to vector, either in GRASS or QGIS (it sounds like you're using both). Following that I would round the z values in the vector to your desired contour levels and then merge these together.

If you've a lot of files to process, it should be possible to automate your workflow with python.

Answer 2

I think this is a job for the QGIS raster calculator, have a look at "Using a Mask", here:

http://spatialgalaxy.net/2012/01/25/using-the-qgis-raster-calculator/

In your case you will only need something like (my_raster@1 >= 3950), don't use the second part of the expression shown.

You could then polygonise the output raster.

N.