polygonqgisqgis-processingrasterzonal statistics
I have the raster map of monthly precipitation of some years (download from CGIAR http://www.cgiar-csi.org/data/uea-cru-ts-v3-10-01-historic-climate-database) and I need the mean values on a specific area (polygonal shapefile). I tried with zonal statistic but I had not results, the attribute table was empty.
Is it possible that my region is too small for do this?
I'm a bit new in QGIS world.
There are several ways to do this. If you don't have Spatial Analyst you can do it anyway as follows:
First, convert the raster to Points using the Raster to Point tool. This gives you a grid of points and is relatively quick - 16million points were created in about 2 minutes (be sure to turn off rendering so they aren't displayed though ;-) ).
Now, use the Analysis Tools -> Overlay - Spatial Join tool. Target feature is your 300 polygon layer, join is the point layer. Leave as one-to-one. Create a "Field Map of Join Features" - being sure to set the merge rule as "mean", "Median", or "mode" (or all of them I guess). Resultant single polygon layer should have all 300 polygon areas, each of which has attributes with the requested types of average calculated in them.
That should get you the numbers you need with minimal effort (though waiting quite a bit of processing - Spatial Joins are slow).
You can use Zonal Statistics as Table (Spatial Analyst) for this type of operation. The tool accepts both vector and raster data as input. You can join the results to your input feature class if you wish.
Best Answer
Before using
Zonal Statisticsmake sure that you have same projections for both raster data and vector (shapefile) data. Sometimes different projection produce empty results. In the following example, I used WGS 84 for both raster data (SRTM Global) and a polygon vector data (Test.shp), and I didn't specify any column prefix:The final output is as follows: