I have some forest polygons and would want to assess the effects the direction (south, north etc) have on vegetation. How do i then determine which forest edge faces which direction either in QGIS or R?
Dear Donald Mwaba First of all you have to calculate ASPECT using a DEM and use raster statistics (mean operator) using your predefine forest polygon as input (zone) to find the mean aspect for each polygon representing forest stand boundaries you have.
Dear @Nasir Hameed and @Gomal Amin , I did calculate the means as advised but how then do I know which one is North and which one is South facing after getting the mean aspect for each polygon?
Check the values in attached file for each aspect class. You can get an idea from here. Let's say if your calculated average aspect is 337 - 360 it mean the direction is north and so on ...
Gomal Amin Ok now that this value is an average for a single polygon, it doesn't tell me which pixels are north facing and which ones are south facing within that polygon right?
Yes, as the mean value was calculated for each polygon it will not provide you pixel wise value. For that you have to convert the raster into polygon but it will be costly in terms of computation.
1. Generate the aspect from dem
2. convert aspect raster to vector
3. Take intersection of Forest Edge vector data with aspect vector data. Now forest edge has the direction value measured from north.
Later, you may convert these values into direction text. E.g. 0degree value represents North.
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