Figure 2.

High-resolution population maps and their use to compute population-weighted distances between counties. The county-level population estimates of Figure 1 were allocated to a grid of 25 km² cells according to the 2000 census block level data. The scattergrams plot Euclidian distances between county geographic centroids, depicted by red dots in the middle maps, versus a "block distance" that accounts for the shape of counties and the distribution of the population (Equation 1). Population-weighted centroids are depicted by blue triangles in the middle maps.