An implementation of Spatial Join MapReduce as described in
S. Zhang, J. Han, Z. Liu, K. Wang, and Z. Xu. SJMR:
Parallelizing spatial join with MapReduce on clusters. In
CLUSTER, pages 1–8, New Orleans, LA, Aug. 2009.
The map function partitions data into grid cells and the reduce function
makes a plane-sweep over each cell.