National Land Cover Data Classification Schemes

"NLCD 1992 was a success, and its success had a lot to do with building the momentum for NLCD 2001. NLCD 2001 improves upon NLCD 1992 in three important ways. Whereas NLCD 1992 was simply a land-cover data set, NLCD 2001 is a land-cover database comprised of three elements: land cover, impervious surface and canopy density. Second, NLCD 2001 used improved classification algorithms, which have resulted in data with more precise rendering of spatial boundaries between the 16 classes (an additional nine classes are available in coastal areas and another four classes in Alaska only). Third, NLCD 2001 also includes Alaska, Hawaii, and Puerto Rico. NLCD 1992 was restricted to the conterminous United States."