A score of years ago geography was characterized as spatial interaction, the underlying concepts of which were complementarity, intervening opportunities, and distance. Without additional consideration of the first two, the last of these concepts is intrinsic to this study. According to Ullman, distance, which may be interpreted in many ways, is measured economically, that is, in terms of the cost and time involved in transporting a product. A good is transportable only when its inherent value is sufficient to overcome the “friction“ of distance. Hoover states this clearly: “When goods of high value per pound are shipped, the transfer charge constitutes a smaller relative addition to the total cost of the delivered article, and such goods are said to be ‘more transportable’ or to be capable of bearing a higher transfer charge.“