Analyzing Raster Data From Vector Layer Buffer

Question

I have satellite images of night-time lights all over earth. I also have a vector shapefile of country boundaries including the largest cities on earth. I want to use buffers to capture a certain amount of light pollution emanating from cities that is viewable from the raster image.

How do I use buffers to analyze pixels (or some aspect of the raster data) within the buffer area? I assume I will want to assign some value to a certain pixel and then total those values by using the buffer to analyze all data within the buffer.

I have attached a snip of what I am working with. I am very new to GIS still, so bear with me.

I am using QGIS 2.0.

Answer 1

I would recommend using the Zonal Statistics Plug-in for QGIS.

With the Zonal Statistics Plugin you can analyze the results of a thematic classification. It allows to calculate several values of the pixels of a raster layer with the help of a polygonal vector layer (see figure_zonal_statistics). You can calculate the sum, the mean value and the total count of the pixels that are within a polygon. The plugin generates output columns in the vector layer with a user-defined prefix.

Or, if you prefer R, you can use Zonal Statistics from the raster package

r <- raster(ncols=10, nrows=10)
r[] <- runif(ncell(r)) * 1:ncell(r)
z <- r
z[] <- rep(1:5, each=20)
# for big files, use a character value rather than a function
zonal(r, z, 'sum')

# for smaller files you can also provide a function

## Not run:
zonal(r, z, mean)
zonal(r, z, min)
## End(Not run)


# multiple layers
zonal(stack(r, r*10), z, 'sum')
  [1]: http://www.qgis.org/en/docs/user_manual/plugins/plugins_zonal_statistics.html