Land cover describes the visible features that cover the Earth's surface, and land cover data are usually derived from satellite imagery or aerial photography. Each pixel on an image will be assigned to a land cover class (e.g., water, vegetation, bare rock, wetlands) through visual interpretation or, more commonly, automated classification algorithms. Researchers may then use the data to quantify the amount of land covered by a particular class(es) or to track changes in land cover over time.
NHGIS land cover summaries use data from the National Land Cover Database, which is produced by a consortium of federal agencies. The NLCD data have a 30-meter spatial resolution and were derived from a decision-tree classification of Landsat satellite imagery. NHGIS provides environmental summaries from the 2001, 2004, 2006, 2008, 2011, 2013, 2016, 2019 and 2021 versions of the NLCD. These nine NLCD versions, collectively known as the legacy NLCD datasets, were created using a consistent methodology and are comparable over time.