The NHGIS environmental summary data files provide land cover data from the National Land Cover Database
(NLCD) summarized over counties and census tracts. This section describes the land cover dataset and the geospatial processing used to generate the county and census tract summaries.
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 uses 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 data were derived from a decision-tree classification of Landsat satellite imagery. Environmental summaries were created from the 2001, 2006, and 2011 versions of the data. These three versions were created using a consistent methodology and are comparable over time.
The NLCD data contains sixteen land cover classes:
Class Value 11. Open water
Areas of open water, generally with less than 25% cover of vegetation or soil.
Class Value 12. Perennial ice/snow
Areas characterized by a perennial cover of ice and/or snow, generally greater than 25% of total cover.
Class Value 21. Developed, open space
Areas with a mixture of some constructed materials, but mostly vegetation in the form of lawn grasses. Impervious surfaces account for less than 20% of total cover. These areas most commonly include large-lot single-family housing units, parks, golf courses, and vegetation planted in developed settings for recreation, erosion control, or aesthetic purposes.
Class Value 22. Developed, low intensity
Areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 20% to 49% of total cover. These areas most commonly include single-family housing units.
Class Value 23. Developed, medium intensity
Areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 50% to 79% of the total cover. These areas most commonly include single-family housing units.
Class Value 24. Developed, high intensity
Highly developed areas where people reside or work in high numbers. Examples include apartment complexes, row houses and commercial/industrial. Impervious surfaces account for 80% to 100% of the total cover.
Class Value 31. Barren land (rock/sand/clay)
Areas of bedrock, desert pavement, scarps, talus, slides, volcanic material, glacial debris, sand dunes, strip mines, gravel pits and other accumulations of earthen material. Generally, vegetation accounts for less than 15% of total cover.
Class Value 41. Deciduous forest
Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75% of the tree species shed foliage simultaneously in response to seasonal change.
Class Value 42. Evergreen forest
Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75% of the tree species maintain their leaves all year. Canopy is never without green foliage.
Class Value 43. Mixed forest
Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. Neither deciduous nor evergreen species are greater than 75% of total tree cover.
Class Value 52. Shrub/scrub
Areas dominated by shrubs; less than 5 meters tall with shrub canopy typically greater than 20% of total vegetation. This class includes true shrubs, young trees in an early successional stage or trees stunted from environmental conditions.
Class Value 71. Grassland/herbaceous
Areas dominated by gramanoid or herbaceous vegetation, generally greater than 80% of total vegetation. These areas are not subject to intensive management such as tilling, but can be utilized for grazing.
Class Value 81. Pasture/hay
Areas of grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops, typically on a perennial cycle. Pasture/hay vegetation accounts for greater than 20% of total vegetation.
Class Value 82. Cultivated crops
Areas used for the production of annual crops, such as corn, soybeans, vegetables, tobacco, and cotton, and also perennial woody crops such as orchards and vineyards. Crop vegetation accounts for greater than 20% of total vegetation. This class also includes all land being actively tilled.
Class Value 90. Woody wetlands
Areas where forest or shrubland vegetation accounts for greater than 20% of vegetative cover and the soil or substrate is periodically saturated with or covered with water.
Class Value 95. Emergent herbaceous wetlands
Areas where perennial herbaceous vegetation accounts for greater than 80% of vegetative cover and the soil or substrate is periodically saturated with or covered with water.