[Qgis-user] How to calculate areas between contour lines (or isolines)

[Micha Silver]

On 20/08/2011 08:42, Jaime Palalane wrote:
> Olás,
>
> Q1: I created contour lines from a DEM and after that I converted them 
> to vector lines. Now, for a basin (catchment) which is a polygon I 
> need to calculate the basin area between specific contour lines ranges 
> (e.g. 300-400). How would appreciate an explanation how it could be 
> done. Another possibility could be to calculate areas between certain 
> contour rages direct from the DEM (my raster file) but I also do not 
> know how to compute them from it.
>
You could create a new raster with unique values for each contour band 
using r.mapcalc, something like:

r.mapcalc ctour_bands="if(dem>300,if (dem<400, 3, null()), if(dem>2, 2, 
null()), null())"

The above should create a raster with category value 3 for everything 
between 300-400 and category 2 for everything between 200-300.
Then you could use r.sum or r.stats -a to get the total area of this 
contour strip.
But chaining together if(...) statements you could create the contour 
band raster with several intervals, and r.stats -a will give the area 
for each category.

> Q2: Is it possible to define catchments which drains to a specific 
> point in QGIS? I managed to create basins using the watershed command 
> but I do not have too much control of the basins which are created and 
> I failed to find how to define the basin draining to a specific 
> section of the river I am studying.
>
Check out the GRASS module r.water.outlet
> Abraço,
>
> Jaime
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