Older landslides (before Hurricane Mitch) in El Salvador

This cover contains the outlines of selected landslides and debris flows that occurred prior to Hurricane Mitch in the areas where hurricane-induced landslides were mapped. The landslides and debris flows shown in this coverage were mapped using 1:40,00-scale and 1:15,000-scale black-and-white aerial photographs. Much of the photography was taken a few months after the storm, but additional photography was taken in late 2000. The photographs were visually examined using 4-X mirror stereoscopes, and the landslides were plotted by hand on mylar overlays that were registered to topographic maps. The landslides were plotted on 1:25,000-scale maps where these high-quality maps were available; for other parts of the country, the landslides were plotted on 1:50,000-scale maps. The plotted landslides were then manually digitized and registered to digital raster graphic (DRG) images of 1:50,000-scale topographic maps. Some of the areas in this coverage are composites of parts of two or more 12:50,000-scale quadrangles. To produce these composites, the edges of adjoining quadrangles had to be digitally adjusted in ArcInfo to assure continuity between adjacent maps. These adjustments were also necessary to resolve differences in the resolution between 1:50,000-scale and 1:25,000-scale topographic maps. After these adjustments, the areas of each plate were cut from merged DRGs and the digital landslides were then plotted on the DRG images of the map areas. The cover portrays the shapes, relative location, and size of pre-hurricane landslides and the associated downslope deposits. For those features that were originally mapped on 1:25,000-scale topographic maps, we estimate that their locations are accurate to within less than 100 m and probably to within 50m. For features that were originally mapped on 1:50,000-scale topographic maps, their locations are probably accurate to within 100-200m.