dividing {HEAD ~ an airspace} encompassing an initial point and {TAIL ~ a destination point} into a plurality of grid cells 
dividing { HEAD ~ an airspace } encompassing an initial point and { TAIL ~ a destination point } into a plurality of grid cells
OTH HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH