I'm using QGIS (2.8.1-Wien) and I have a raster layer showing chlorophyll concentrations in the ocean. I have also defined number of regions of the ocean using polygons. What I'd like to calculate for each region is the area within that polygon where the raster is greater than X. For example, what area in each polygon has a chlorophyll concentration greater than 2. Any suggestions?
Here's my solution. I used the
Raster Calculator to determine where my raster is greater than X, outputting a layer that is 1 (greater than X) or 0 (less than or equal to X). I then used the
Zonal Statistics plugin to calculate the mean value of this new layer within each polygon. This gives the proportion of the polygon where the raster is greater than X. Multiplying by the area of my polygons gives the area in each polygon where the raster is greater than X.