This product is a 100-meter resolution raster which classifies urban lands in the conterminous United States by the year in which they were “first developed.” The classification is based on housing density by era for 1940-2010 developed by the University of Wisconsin-Madison SILVIS lab. The classification was applied by identifying a housing unit density threshold which matched current-era “development” as represented by U.S. Census Urban Areas polygons, then re-applying that threshold to former eras back to 1940. The result is a national raster (.tif format) with eight classes, for the years 1940 and before, 1950, 1960, 1970, 1980, 1990, 2000, and 2010, representing the year in which a pixel crossed the threshold of being “developed.” It is suitable for watershed mean calculations in distinguishing areas which have newly developed areas vs. older developed areas.