In the past decade the question of the degree of mutual economic dependence among the three major regions of the antebellum United States—Northeast, South, and West—has received considerable discussion. Good indicators of the degree of dependence would be ratios of the interregional trade flows of particular commodities to the amounts of those commodities produced locally in each region. Trade data are available, however, only in limited series for major cities or commercial areas. Since comprehensive trade flows cannot be measured directly, various indirect approaches to the question have been employed.
Ionospheric tomography via iterative cross-entropy minimization