The author would like to thank David Singleton, Lindsay Hood, Steve Roberts and Stuart Ramsden for many thought provoking conversations on the subject of numerical methods and parallel computers, Terry Bossameier for his proof reading and encouragement, and David Stewart for his expert advice on suitable numerical methods for interpolating on a sphere. Hans Moravec set me thinking about this particular finite-difference like approach during e-mail discussion regarding his wave based rendering method [10][11].