1. von Thünen, J. H.: Der isolierte Staat in Beziehung auf Landwirtschaft und Nationalökonomie. Rostock 1826, quoted in Chisholm, M.: Rural Settlement and Land Use, 1966
2. The gravity model has been used in a number of transport geography papers including Helvig, M.: Chicago's External Truck Movements: Special Interactions between the Chicago Area and its Hinterland. Research paper 90, Department of Geography, Chicago University (1964).
3. Haggett, P.: Network Models in Geography. In: Chorley, R. J.; Haggett, P.; Models in Geography. pp. 609?664, 1967
4. Taaffe, H. J.; Morrill, R. L.; Gould, P. R.: Transport Expansion in Underdeveloped Countries: A comparative Analysis. The Geographical Review, 53, 503?529 (October 1963)
5. Haggett, P., op. cit. pp. 609?664