Figure 1.

(Left) Joint distribution of human population by elevation and population density, or Human Altitude Density (HAD), as a contour map. Contour lines show log₁₀(people). Example: the contour curve on the left that is labeled 8 passes through combinations of elevation and population density where about 10⁸ people live. Horizontal bin size is linear in elevation with a linear increment of 100 m, and vertical bin size is linear in log₁₀(people/km²), with each bin covering a constant factor of 10^0.25 (= 1.78) in population density. Thus 4 vertical bins span a 10-fold increase in population density. (Right) Joint distribution of land area by elevation and population density, or Land Altitude Density (LAD), as a contour map. Contour lines show log₁₀(land area (in km²)). For example, there are about 10⁶ km² of occupied land at each combination of elevation and population density through which the contour curve on the left that is labeled 6 passes. In both distributions, the values falling outside the bounds of the histogram are accumulated in the peripheral bins.