Using over() function in R with Polygons and Raster

Question

I want to calculate the average rain by region using the over function in R but I am getting the error "unable to find an inherited method for function over for signature RasterLayer, SpatialPolygonsDataFrame". This is my code:

rain <- readOGR(dsn="/Users___.shp", layer="rain")
class(rain)
#SpatialPolygonsDataFrame
region <- readOGR(dsn="/Users___.shp", layer="region")
class(region)
#SpatialPolygonsDataFrame
s <- raster(ncol=90, nrow=90)
extent(s) <- extent(rain)
rain_raster <- rasterize(rain, s)
class(rain_raster)
#RasterLayer
test1 <- over(rain_raster, region, fn="mean")
#error: unable to find an inherited method for function ‘over’ for signature ‘"RasterLayer", "SpatialPolygonsDataFrame"

How can I calculate this average without getting this error?

Answer 1

over doesn't do raster-polygon computations. You need something like extract from the raster package:

This will return the mean of all raster cells in each part of region:

 test1 <- raster::extract(rain_raster, region, fun=mean)

• Amy: I tried using extract but I am getting an error.
• Spacedman: If extract doesn't work, please edit your question and show us the problem there.