The National Land Cover Database 2001 Alaska Land Cover Cloud/Shadow Mask for the state of Alaska was produced through a cooperative project conducted by the Multi-Resolution Land Characteristics (MRLC) Consortium. The MRLC Consortium is a partnership of federal agencies (www.mrlc.gov), consisting of the U.S. Geological Survey (USGS), the National Oceanic and Atmospheric Administration (NOAA), the U.S. Environmental Protection Agency (EPA), the U.S. Department of Agriculture (USDA), the U.S. Forest Service (USFS), the National Park Service (NPS), the U.S. Fish and Wildlife Service (FWS), the Bureau of Land Management (BLM) and the USDA Natural Resources Conservation Service (NRCS). One of the primary goals of the project is to generate a current, consistent, seamless, and accurate National Land cover Database (NLCD) circa 2001 for the United States at medium spatial resolution. This landcover map and all documents pertaining to it are considered "provisional" until a formal accuracy assessment can be conducted. For a detailed definition and discussion on MRLC and the NLCD 2001 products, refer to Homer et al. (2004) and http://www.mrlc.gov/mrlc2k.asp.
The NLCD 2001 is created by partitioning the U.S. into mapping zones. A total of 66 mapping zones within the conterminous U.S., with an additional 12 in Alaska, were delineated based on ecoregion and geographical characteristics, edge matching features and the size requirement of Landsat mosaics. The Alaska Land Cover Cloud/Shadow Mask data layer covers the entire state of Alaska. Questions about the NLCD 2001 Alaska Land Cover Cloud/Shadow Mask data layer can be directed to the NLCD 2001 land cover mapping team at the USGS/EROS, Sioux Falls, SD (605) 594-6151 or mrlc@usgs.gov.
The Alaska Land Cover Cloud/Shadow Mask data layer is a companion layer to the NLCD Alaska 2001 Land Cover and was developed to provide additional information regarding land cover extrapolation in areas covered by clouds, cloud shadows, or terrain shadows on the Landsat imagery used for land cover classification. This binary mask is provided to alert data end users that land cover in areas identified by the mask may have been classified with limited or no spectral information. The abbreviated growing season and areas of sporadic, or perennial cloud cover, frequently limit the selection of acceptable quality Landsat scenes in Alaska. Topographic features that create shadowed areas during the time of Landsat overpass also impact scene selection choices. In some instances clouds, cloud shadows and/or terrain shadows were present in all available scenes for a path/row, in which case the best possible scene was chosen. Regardless of scene quality, an attempt was made to map land cover for every pixel in the state even though the Landsat imagery may have had reduced quality or was unusable. These areas have been categorized into land cover types using the best available methodology, but in some cases may have been classified using only elevation and elevation derivatives like slope, aspect, and positional index. Pixels that were classified using poor quality, or no spectral information are indicated in the mask with a value of 1.
