Topographic Accessability Index

The topographic accessibility index is a measure of elevation in relation to valley floor corrected for variation in valley floor elevation across the western United States (i.e., valley floor elevation at California's coast is much lower compared to a valley floor in Wyoming). This index was based on 90-m resolution digital elevation model grids (DEM; National Elevation Dataset, USGS EROS [Earth Resources Observation Systems]),3 and a landform model of the western United States (Manis et al. 2001), which delineates valley flats and near level plateaus or terraces (cell values, 1 or 4). We reclassified the landform model (cell value, 1) and multiplied this layer by the DEM to derive valley floor elevation. Using a moving window analysis (303 x 303 cells; 743.65 km2), we computed mean regional valley floor elevations and subtracted them from the DEM, to derive the difference between the local elevation and regional valley floor elevation. This data set was then resampled to 180 m resolution for analysis.