Estimating Population, Homes and Land Risks

UPDATE: For areas outside the U.S., population analysis employs 2010 LandScan data.

To calculate potential risks at each water level within towns, cities, counties or states, we used boundaries provided by the 2010 U.S. Census to overlay against our maps of land beneath different water level thresholds (see methods for map development).  We then computed the amount of land below each threshold in each place, for both total land and dry land (excluding freshwater wetlands).  For denominators in percentage calculations, we used our own computations of land area for each place, because our definitions of coastline differed slightly in places from that of the Census (see related methods).

To tabulate population and housing potentially affected, we used block-level data from the 2010 U.S. Census, and assumed development on dry land only. For each Census block, we divided the population and number of housing units by the number of dry land cells with centers inside the block. We assigned the resulting per-cell density values back to each cell, creating new datasets for population and housing unit density. To estimate the population or housing at risk for a particular water level, we simply added up population and housing densities of land cells affected under the specification. Our analysis considered the elevation of land upon which housing stands, and made no special provision for elevated or multi-story buildings.

For a more detailed description of our methods, including discussion of the expected accuracy of land, population and housing estimates, see our published paper.