Land cover is the physical material that covers the surface of the earth – vegetation, water, soil, and buildings. This indicator reports how land cover has changed in the Triangle between 1992 and 2001, the latest period for which data are available. This indicator uses satellite data to show the amount the Triangle Region covered by water, urban development, forest, grass- and shrub-land, agriculture, and wetlands (see Technical Note for details) in 1992 and 2001. From these data we can also create a land cover change matrix that shows which land covers have been converted to other land covers between 1992 and 2001.
Why is this important?
The amount of land covered by different materials is a basic characteristic of landscapes and determines in part how our ecological systems function. For example, forests store large amounts of carbon so changes in the amount of forest cover can reflect significant changes in carbon storage and release. Impervious surfaces, such as pavement and building roofs, intercept rainwater and can change the degree to which groundwater recharge occurs and the level of pollutants entering streams.
Figure 1. Triangle land cover in 1992.
Figure 2. Triangle land cover in 2001.
What does this measure show?
While the region is still predominantly covered by forest (47%) and agriculture (22%), we are converting large area of forest to urban cover, which now covers 16% of the Triangle (Figure 2, Table 1, Table 2).
Most of the change was in urban cover, which increased 11% from 471 to 525 mi2, and forested land, which decreased 7% from 1,669 to 1,553 mi2. Grass- and shrub-land also increased, largely as a result of forest clearing. Most of the land developed during this period was forested in 1992 (48 mi2), with the rest coming from agricultural land (9 mi2).
Table 1. Land Cover, 1992-2001. Area is shown in square miles. All values rounded to whole numbers.
Table 2.Land cover change matrix, 1992-2001, in square miles. All values rounded to whole numbers. This table can be used to determine how land use changed among categories between 1992 and 2001. Read across the rows to find area converted from a category to new land uses. Read down the columns to see the sources of land converted to a different land cover. For example, of the 1,669 mi2 of forest in 1992, reading across the forest row shows that in 2001, 1,523 mi2 were still forested; 38 mi2 had been converted to agriculture; 1 mi2 to barren; 54 mi2 to grass/shrub; and so forth across the row. Reading down the forest column shows that 30 mi2 of land that was agriculture in 1992 became forested by 2001.
Limitations
The 2001 data are the newest available - and they are 9 years old as of this writing. In a rapidly changing area like the Triangle, much will have changed during those 9 years - more fields and forests will have been converted to built landscapes. A 2006 version is currently being created, but as of 2010 April 18 there is still no release date available.
AuthorGeorge Hess, NC State University :: 2010 April 6
Technical Notes
These statistics were generated from the Multi-Resolution Land Characteristic Consortium's Land Cover Change Retrofit product, produced by the US Geological Survey. The Land Cover Change Retrofit product documented in Fry, J.A., Coan, M.J., Homer, C.G., Meyer, D.K., and Wickham, J.D. 2009. Completion of the National Land Cover Database (NLCD) 1992–2001 Land Cover Change Retrofit product: U.S. Geological Survey Open-File Report 2008–1379, 18 p. available at http://pubs.usgs.gov/of/2008/1379/pdf/ofr2008-1379.pdf on 2010 Apr 18. We've uploaded a copy here: Fry-etal2009_NLCD_Change.pdf
The Multi-Resolution Land Characteristics (MRLC) Consortium is a group of federal agencies who first joined together in 1993 (MRLC 1992 ) to purchase Landsat 5 imagery for the conterminous U.S. and to develop a land cover dataset called the National Land Cover Dataset (NLCD 1992) . In 1999, a second-generation MRLC consortium (see logos) was formed to purchase three dates of Landsat 7 imagery for the entire United States (MRLC 2001 ) and to coordinate the production of a comprehensive land cover database for the nation called the National Land Cover Database (NLCD 2001) . (Copied from http://www.mrlc.gov/about.php 2010 Apr 18)
Agriculture - including cultivated crops and pasture/hay – Cultivated crops are described as 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. This class also includes all actively tilled land. Pasture/Hay is described as grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops, typically on a perennial cycle. Pixels coded to a value of 6 have not changed between 1992 and 2001.
Barren - Barren 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. Pixels coded to a value of 3 have not changed between 1992 and 2001.
Forest - Areas dominated by trees generally taller than 5 meters, and greater than 20% of total vegetation cover. Includes deciduous forest, evergreen forest, and mixed forest. Pixels coded to a value of 4 have not changed between 1992 and 2001.
Grassland/Shrub - Includes grassland areas dominated by gramminoid or herbaceous vegetation and shrub/scrub areas dominated by shrubs less than 5 meters tall with shrub canopy typically greater than 20% of total vegetation, including true shrubs, young trees in an early successional stage, or trees stunted due to harsh environmental conditions. Management techniques that associate soil, water, and forage-vegetation resources are more suitable for rangeland management than are practices generally used in managing pastureland. Some rangelands have been or may be seeded to introduced or domesticated plant species. Includes those areas in the Eastern United States that commonly are called brushlands. Pixels coded to a value of 5 have not changed between 1992 and 2001 (Anderson, et al., 1976).
Ice/Snow (not present in the Triangle) - All areas characterized by a perennial cover of ice and/or snow, generally greater than 25% of total cover. Pixels coded to a value of 8 have not changed between 1992 and 2001.
Open Water - All areas of open water, generally with less than 25% vegetation or soil cover. Pixels coded to a value of 1 have not changed between 1992 and 2001.
Urban - Includes developed open spaces with a mixture of some constructed materials, but mostly vegetation in the form of lawn grasses such as large-lot single-family housing units, parks, golf courses, and vegetation planted in developed settings for recreation, erosion control, or aesthetic purposes. Also included are lands of low, medium, and high intensity with a mixture of constructed materials and vegetation, such as single-family housing units, multifamily housing units, and areas of retail, commercial, and industrial uses. Pixels coded to a value of 2 have not changed between 1992 and 2001.
Wetlands - including woody wetlands and herbaceous wetlands – Areas where forest or shrubland vegetation accounts for greater than 20 percent of vegetative cover and the soil or substrate is periodically saturated with or covered with water. This class also includes areas where perennial herbaceous vegetation accounts for greater than 80 percent of vegetative cover and the soil or substrate is periodically saturated with or covered with water. Pixels coded to a value of 7 have not changed between 1992 and 2001.
Land Cover Change
What is this?
Land cover is the physical material that covers the surface of the earth – vegetation, water, soil, and buildings. This indicator reports how land cover has changed in the Triangle between 1992 and 2001, the latest period for which data are available. This indicator uses satellite data to show the amount the Triangle Region covered by water, urban development, forest, grass- and shrub-land, agriculture, and wetlands (see Technical Note for details) in 1992 and 2001. From these data we can also create a land cover change matrix that shows which land covers have been converted to other land covers between 1992 and 2001.
Why is this important?
The amount of land covered by different materials is a basic characteristic of landscapes and determines in part how our ecological systems function. For example, forests store large amounts of carbon so changes in the amount of forest cover can reflect significant changes in carbon storage and release. Impervious surfaces, such as pavement and building roofs, intercept rainwater and can change the degree to which groundwater recharge occurs and the level of pollutants entering streams.
What does this measure show?
While the region is still predominantly covered by forest (47%) and agriculture (22%), we are converting large area of forest to urban cover, which now covers 16% of the Triangle (Figure 2, Table 1, Table 2).
Most of the change was in urban cover, which increased 11% from 471 to 525 mi2, and forested land, which decreased 7% from 1,669 to 1,553 mi2. Grass- and shrub-land also increased, largely as a result of forest clearing. Most of the land developed during this period was forested in 1992 (48 mi2), with the rest coming from agricultural land (9 mi2).
Table 1. Land Cover, 1992-2001. Area is shown in square miles. All values rounded to whole numbers.
Table 2. Land cover change matrix, 1992-2001, in square miles. All values rounded to whole numbers. This table can be used to determine how land use changed among categories between 1992 and 2001. Read across the rows to find area converted from a category to new land uses. Read down the columns to see the sources of land converted to a different land cover. For example, of the 1,669 mi2 of forest in 1992, reading across the forest row shows that in 2001, 1,523 mi2 were still forested; 38 mi2 had been converted to agriculture; 1 mi2 to barren; 54 mi2 to grass/shrub; and so forth across the row. Reading down the forest column shows that 30 mi2 of land that was agriculture in 1992 became forested by 2001.
Limitations
The 2001 data are the newest available - and they are 9 years old as of this writing. In a rapidly changing area like the Triangle, much will have changed during those 9 years - more fields and forests will have been converted to built landscapes. A 2006 version is currently being created, but as of 2010 April 18 there is still no release date available.
Author George Hess, NC State University :: 2010 April 6
Technical Notes
These statistics were generated from the Multi-Resolution Land Characteristic Consortium's Land Cover Change Retrofit product, produced by the US Geological Survey. The Land Cover Change Retrofit product documented in Fry, J.A., Coan, M.J., Homer, C.G., Meyer, D.K., and Wickham, J.D. 2009. Completion of the National Land Cover Database (NLCD) 1992–2001 Land Cover Change Retrofit product: U.S. Geological Survey Open-File Report 2008–1379, 18 p. available at http://pubs.usgs.gov/of/2008/1379/pdf/ofr2008-1379.pdf on 2010 Apr 18. We've uploaded a copy here: Fry-etal2009_NLCD_Change.pdf
The Multi-Resolution Land Characteristics (MRLC) Consortium is a group of federal agencies who first joined together in 1993 (MRLC 1992 ) to purchase Landsat 5 imagery for the conterminous U.S. and to develop a land cover dataset called the National Land Cover Dataset (NLCD 1992) . In 1999, a second-generation MRLC consortium (see logos) was formed to purchase three dates of Landsat 7 imagery for the entire United States (MRLC 2001 ) and to coordinate the production of a comprehensive land cover database for the nation called the National Land Cover Database (NLCD 2001) . (Copied from http://www.mrlc.gov/about.php 2010 Apr 18)
Definitions of cover classes from http://www.mrlc.gov/changeproduct_definitions.php on 2010 Apr 18
Here is the spreadsheet containing the data: TriangeLandCoverChange.xls
Indicators:: Population Trends :: Urbanized Area Growth :: Land Cover Change