A function to overlay the FFT grid, a SpatialPolygons object, onto a SpatialPolygonsDataFrame object.
gOverlay(grid, spdf)
the FFT grid, a SpatialPolygons object
a SpatialPolygonsDataFrame object
a matrix describing the features of the overlay: the originating indices of grid and spdf (all non-trivial intersections) and the area of each intersection.
this code was adapted from Roger Bivand: https://stat.ethz.ch/pipermail/r-sig-geo/2011-June/012099.html