The function coarsenGrid is a function that samples from a
SpatialGridDataFrame.
The argument coarse indicates that every coarse row and column will
be sampled, starting with the row and column represented in offset. offset = c(0,0)
implies that the smallest x- and y-coordinates will be a part of the resampled
data set, offset = c(1,1) implies that sampling will start on the second row and column.
References
Pebesma, E., Cornford, D., Dubois, G., Heuvelink, G.B.M., Hristopulos, D., Pilz, J., Stohlker, U., Morin, G., Skoien, J.O. INTAMAP: The design and implementation f an interoperable automated interpolation Web Service. Computers and Geosciences 37 (3), 2011.