The National Land Cover Characterization project is part of the interagency Multi-Resolution Land Characterization (MRLC) initiative involving three divisions of the US Geological Survey (National Mapping Division, Water Resources Division, and the Biological Resources Division), the Environmental Protection Agency, the National Oceanic and Atmospheric Administration, and the US Forest Service. These agencies have a requirement for a nationally consistent land cover data set, and in 1993 these agencies combined financial resources to purchase Landsat Thematic Mapper data covering the entire United States. The USGS/EROS Data Center took the lead to process the data and develop the National Land Cover Data (NLCD) set. The classification system for individual state NLCD rasters contain 21 different land cover categories, each with a spatial resolution of 30 meters. This National grid of the NLCD was generated for use with the GeoStac database. It has been generated by merging the state NLCD data sets into a single US-wide NLCD dataset, and then generating individual US-wide subsets of the 21 different 1992 NLCD land cover categories. Each US-wide subset maintains a spatial resolution of 30 meters.

The main objective of this project was to generate a generalized and nationally consistent land cover data layer for the entire conterminous United States. These data can be used as a layer in a geographic information system (GIS) for any number of purposes such assessing wildlife habitat, water quality and pesticide runoff, land use change, etc. For use with the GeoStac database, the US-wide NLCD subsets have been generated in order to simplify pesticide risk assesement and to provide a common data set upon which to perform anaylsis for all stakeholders.

The US-wide NLCD subsets are 1-bit (binary) ESRI grid files; therefore, for each US-Wide NLCD subset, the 1992 state NLCD class number has been changed to 1 in the Value field of the attributes table.

Users must have a firm understanding of how the datasets were compiled and the resulting limitations of these data. The National Land Cover Dataset was compiled from Landsat satellite TM imagery (circa 1992) with a spatial resolution of 30 meters and supplemented by various ancillary data (where available). The analysis and interpretation of the satellite imagery was conducted using very large, sometimes multi-state image mosaics (i.e. up to 18 Landsat scenes). Using a relatively small number of aerial photographs for 'ground truth', the thematic interpretations were necessarily conducted from a spatially-broad perspective. Furthermore, the accuracy assessments correspond to 'federal regions' which are groupings of contiguous states. Thus, the reliability of the data is greatest at the state or multi-state level. The statistical accuracy of the data is known only for the region. Important Caution Advisory With this in mind, users are cautioned to carefully scrutinize the data to see if they are of sufficient reliability before attempting to use the dataset for larger-scale or local analyses. This evaluation must be made remembering that the NLCD represents conditions in the early 1990s.

An accuracy assessment is done on all NLCD on a Federal Region basis following a revision cycle that incorporates feedback from MRLC Consortium partners and affiliated users. The accuracy assessments are conducted by private sector vendors under contract to the US EPA. A protocol has been established by the USGS and US EPA that incorporates a two-stage, geographically stratified cluster sampling plan (Zhu et al., 1999) utilizing National Aerial Photography Program (NAPP) photographs as the sampling frame and the basic sampling unit. In this design a NAPP photograph is defined as a 1st stage or primary sampling unit (PSU), and a sampled pixel within each PSU is treated as a 2nd stage or secondary sampling unit (SSU). PSU's are selected from a sampling grid based on NAPP flight-lines and photo centers, each grid cell measures 15' X 15' (minutes of latitude/longitude) and consists of 32 NHAP photographs. A geographically stratified random sampling is performed with 1 NAPP photo being randomly selected from each cell (geographic strata), if a sampled photo falls outside of the regional boundary it is not used. Second stage sampling is accomplished by selecting SSU's (pixels) within each PSU (NAPP photo) to provide the actual locations for the reference land cover classification. The SSU's are manually interpreted and misclassification errors are estimated and described using a traditional error matrix as well as a number of other important measures including the overall proportion of pixels correctly classified, user's and producer's accuracy's, and omission and commission error probabilities. At the time of CD release (Summer 2000), the accuracy assessment was not complete. For the Region IV accuracy assessment, please check the NLCD Website: http://edcwww.usgs.gov/programs/lccp/nationallandcover.html. The accuracy assessment numbers will be posted there around September, 2000. While we believe that the approach taken has yielded a very good general land cover classification product for Region IV, it is important to indicate to the user where there might be some potential problems. The biggest concerns for Region IV are listed below: 1) Accurate definition of the transitional barren class was extremely difficult. The majority of pixels in this class correspond to clear-cut forests in various stages of regrowth. Spectrally, fresh clear-cuts are very similar to row-crops in the leaves-off data. Manual correction of coding errors was performed to improve differentiation between row-crops and clear-cuts, but some errors may still be found. As regrowth occurs in a clear-cut region, the definition of transitional barren versus a forested class becomes problematic. An attempt was made to classify only fresh clear-cuts or those in the earliest stages of regrowth, but there are likely forested regions classed as transitional barren and vice versa. 2) Due to the confusion between clear-cuts, regrowth in clear-cuts, Forested areas, and shrublands, no attempts were made to populate the shrubland classes. Any shrubland areas that exist in this area are classed in their like forest class, i.e. deciduous shrubland is classed as deciduous forest, etc.

For the 1992 state NLCD data sets, an unsupervised classification algorithm was used to classify the mosaicked multiple leaf-off TM scenes. Aerial photographs were used to interpret and label classes into land cover categories and ancillary data sources resolved the class confusion. Further land cover information from leaf-on TM data, NWI data, and other sources were incorporated to refine and augment the "basic" classification.

For the state NLCD data sets, each Landsat Thematic Mapper image used to create the NLCD was precision terrain-corrected using 3-arc-second digital terrain elevation data (DTED), and georegistered using ground control points. This resulted in a root mean square registration error of less than 1 pixel (30 meters).

Process_Description:

Land Cover Characterization:
The project is being carried out on the basis of 10 Federal Regions
that make up the conterminous United States; each region is comprised
of multiple states; each region is processed in subregional units
that are limited to the area covered by no more than 18 Landsat TM
scenes. The general NLCD procedure is to: (1) mosaic subregional TM
scenes and classify them using an unsupervised clustering algorithm,
(2) interpret and label the clusters/classes using aerial photographs
as reference data, (3) resolve the labeling of confused clusters/classes
using the appropriate  ancillary data source(s), and (4) incorporate
land cover information from other data sets and perform manual edits to
augment and refine the "basic" classification developed above.

Two seasonally distinct TM mosaics are produced, a leaves-on version
(summer) and a leaves-off (spring/fall) version.  TM bands 3, 4, 5,
and 7 are mosaicked for both the leaves-on and leaves-off versions.
For mosaick purposes, a base scene is selected for each mosaic and
the other scenes are adjusted to mimic spectral properties of the base
scene using histogram matching in regions of spatial overlap.
Following mosaicking, either the leaves-off version or leaves-on version
Is selected to be the "base" for the land cover mapping process.  The 4
TM bands of the "base" mosaic are clustered to produce a single 100-
class image using an unsupervised clustering algorithm. Each of the
spectrally distinct clusters/classes is then assigned to one or more
Anderson level 1 and 2 land cover classes using National High Altitude
Photography program (NHAP)and National Aerial Photography program
(NAPP) aerial photographs as a reference.  Almost invariably, individual
spectral clusters/classes are confused between two or more land cover
classes.

Separation of the confused spectral clusters/classes into appropriate
NLCD class is accomplished using ancillary data layers.  Standard
ancillary data layers include: the "non-base" mosaic TM bands and 100-
class cluster image; derived TM normalized vegetation index (NDVI),
various TM band ratios, TM date bands; 3-arc second Digital Terrain
Elevation Data (DTED) and derived slope, aspect and shaded relief;
population and housing density data; USGS land use and land cover
(LUDA); and National Wetlands Inventory(NWI) data if available. Other
ancillary data sources may include soils data, unique state or regional
land cover data sets, or data from other federal programs such as the
National Gap Analysis Program (GAP) of the USGS Biological Resources
Division (BRD).  For a given confused spectral cluster/class, digital
values of the various ancillary data layers are compared to determine:
(1) which data layers are the most effective for splitting the
confused cluster/class into the appropriate NLCD class, and (2) the
appropriate layer thresholds for making the split(s).  Models are then
developed using one to several ancillary data layers to split the
confused cluster/class into the NLCD class.  For example, a population
density threshold is used to separate high-intensity residential areas
from commercial/industrial/transportation.  Or a cluster/class might be
confused between row crop and grasslands.  To split this particular
cluster/class, a TM NDVI threshold might be identified and used with an
elevation threshold in a class-splitting model to make the appropriate
NLCD class assignments.  A purely spectral example is using the
temporally opposite TM layers to discriminate confused cluster/classes
such as hay pasture vs. row crops and deciduous forests vs. evergreen
forests; simple thresholds that contrast the seasonal differences in
vegetation between leaves-on vs. leaves-off.

Not all cluster/class confusion can be successfully modeled out.
Certain classes such as urban/recreational grasses or quarries/strip
mines/gravel pits that are not spectrally unique require manual editing.
These class features are typically visually identified and then
reclassified using on-screen digitizing and recoding.  Other classes
such as wetlands require the use of specific data sets such as NWI to
provide the most accurate classification.  Areas lacking NWI data are
typically subset out and modeling is used to estimate wetlands in these
localized areas.  The final NLCD product results from the classification
(interpretation and labeling) of the 100-class "base" cluster mosaic
using both automated and manual processes, incorporating both spectral
and conditional data layers.  For a more detailed explanation please
see Vogelmann et al. 1998 and Vogelmann et al. 1998.

Discussion:

While we believe that the approach taken has yielded a very good general
land cover classification product for the nation, it is important to
indicate to the user where there might be some potential problems.  The
biggest concerns are listed below:

1) Some of the TM data sets are not temporally ideal. Leaves-off data
sets are heavily relied upon for discriminating between hay/pasture and
row crop, and also for discriminating between forest classes.  The
success of discriminating between these classes using leaves-off data
sets hinges on the time of data acquisition.  When hay/pasture areas are
non-green, they are not easily distinguishable from other agricultural
areas using remotely sensed data.  However, there is a temporal window
during which hay and pasture areas green up before most other vegetation
(excluding evergreens, which have different spectral properties); during
this window these areas are easily distinguishable from other crop
areas. The discrimination between hay/pasture and deciduous forest is
likewise optimized by selecting data in a temporal window  where
deciduous vegetation has yet to leaf out. It is difficult to acquire a
single-date of imagery (leaves-on or leaves-off) that adequately
differentiates between both deciduous/hay and pasture and hay pasture
/row crop.

2) The data sets used cover a range of years (see data sources), and
changes that have taken place across the landscape over the time period
may not have been captured.  While this is not viewed as a major problem
for most classes, it is possible that some land cover features change
more rapidly than might be expected (e.g. hay one year, row crop the
next).

3)  Wetlands classes are extremely difficult to extract from Landsat TM
spectral information alone.  The  use of ancillary information such as
National Wetlands Inventory (NWI) data is highly desirable.  We relied
on GAP, LUDA, or proximity to streams and rivers as well as spectral
data to delineate wetlands in areas without NWI data.

4) Separation of natural grass and shrub is problematic. Areas observed
on the ground to be shrub or grass are not always distinguishable
spectrally. Likewise, there was often disagreement between LUDA and GAP
on these classes.

Acknowledgments

This work was performed under contract the U.S. Geological
Survey (Contract 1434-CR-97-CN-40274).

References

More detailed information on the methodologies and techniques employed
In this work can be found in the following:

Kelly, P.M., and White, J.M., 1993. Preprocessing remotely sensed data
for efficient analysis and classification, Applications of Artificial
Intelligence 1993: Knowledge-Based Systems in Aerospace and Industry,
Proceeding of SPIE, 1993, 24-30.

Cowardin, L.M., V. Carter, F.C. Golet, and E.T. LaRoe, 1979.
Classification of Wetlands and Deepwater Habitats of the United States,
Fish and Wildlife Service, U.S. Department of the Interior, Washington,
D.C.

Vogelmann, J.E., Sohl, T., and Howard, S.M., 1998. "Regional
Characterization of Land Cover Using Multiple Sources of Data."
Photogrammetric Engineering & Remote Sensing, Vol. 64, No. 1,
pp. 45-47.

Vogelmann, J.E., Sohl, T., Campbell, P.V., and Shaw, D.M., 1998.
"Regional Land Cover Characterization Using Landsat Thematic Mapper
Data and Ancillary Data Sources." Environmental Monitoring and
Assessment, Vol. 51, pp. 415-428.

Zhu, Z., Yang, L., Stehman, S., and Czaplewski, R., 1999. "Designing an
Accuracy Assessment for USGS Regional Land Cover Mapping Program."
(In review) Photogrametric Engineering & Remote Sensing.
This process was performed by the US Geologic Survey.

Raw NLCD datasets were acquired in ERDAS Imagine format. The individual state datasets were merged into a single US-wide NLCD dataset using the ERDAS Imagine Mosaic Tool (Data Prep | Mosaic Images) with the following parameters: Input Images: Individual NLCD images for conterminous US (48 states and Washington, DC) Image Matching: Matching Method - No Matching Pixel Value Type - Pixel Value Overlap Function: Intersection Type - No Cutline Select Function - Maximum Output Options: Output Map Area - Union of All Inputs Output Data Type - Unsigned 8-bit From the compiled US-wide 1992 NLCD raster image, subsets of the 21 different 1992 NLCD land use classes were generated using the models NLCDSubsets.gmd resulting in 1-bit (binary) output files for each class.

From the original source 1992 state NLCD data sets, the following defines the 21 different NLCD classes: Water 11 Open Water 12 Perennial Ice/Snow Developed 21 Low Intensity Residential 22 High Intensity Residential 23 Commercial/Industrial/Transportation Barren 31 Bare Rock/Sand/Clay 32 Quarries/Strip Mines/Gravel Pits 33 Transitional Vegetated; Natural Forested Upland 41 Deciduous Forest 42 Evergreen Forest 43 Mixed Forest Shrubland 51 Shrubland Non-natural Woody 61 Orchards/Vineyards/Other Herbaceous Upland 71 Grasslands/Herbaceous Herbaceous Planted/Cultivated 81 Pasture/Hay 82 Row Crops 83 Small Grains 84 Fallow 85 Urban/Recreational Grasses Wetlands 91 Woody Wetlands 92 Emergent Herbaceous Wetlands NLCD Land Cover Classification System Land Cover Class Definitions: Water - All areas of open water or permanent ice/snow cover. 11. Open Water - areas of open water, generally with less than 25 percent or greater cover of water (per pixel). 12. Perennial Ice/Snow - All areas characterized by year-long cover of ice and/or snow. Developed - areas characterized by high percentage (approximately 30% or greater) of constructed materials (e.g. asphalt, concrete, buildings, etc). 21. Low Intensity Residential - Includes areas with a mixture of constructed materials and vegetation. Constructed materials account for 30-80 percent of the cover. Vegetation may account for 20 to 70 percent of the cover. These areas most commonly include single-family housing units. Population densities will be lower than in high intensity residential areas. 22. High Intensity Residential - Includes heavily built up urban centers where people reside in high numbers. Examples include apartment complexes and row houses. Vegetation accounts for less than 20 percent of the cover. Constructed materials account for 80-100 percent of the cover. 23. Commercial/Industrial/Transportation - Includes infrastructure (e.g. roads, railroads, etc.) and all highways and all developed areas not classified as High Intensity Residential. Barren - Areas characterized by bare rock, gravel, sad, silt, clay, or other earthen material, with little or no "green" vegetation present regardless of its inherent ability to support life. Vegetation, if present, is more widely spaced and scrubby than that in the "green" vegetated categories; lichen cover may be extensive. 31. Bare Rock/Sand/Clay - Perennially barren areas of bedrock, desert, pavement, scarps, talus, slides, volcanic material, glacial debris, and other accumulations of earthen material. 32. Quarries/Strip Mines/Gravel Pits - Areas of extractive mining activities with significant surface expression. 33. Transitional - Areas of sparse vegetative cover (less than 25 percent that are dynamically changing from one land cover to another, often because of land use activities. Examples include forest clearcuts, a transition phase between forest and agricultural land, the temporary clearing of vegetation, and changes due to natural causes (e.g. fire, flood, etc.) Forested Upland - Areas characterized by tree cover (natural or semi-natural woody vegetation, generally greater than 6 meters tall); Tree canopy accounts for 25-100 percent of the cover. 41. Deciduous Forest - Areas dominated by trees where 75 percent or more of the tree species shed foliage simultaneously in response to seasonal change. 42. Evergreen Forest - Areas characterized by trees where 75 percent or more of the tree species maintain their leaves all year. Canopy is never without green foliage. 43. Mixed Forest - Areas dominated by trees where neither deciduous nor evergreen species represent more than 75 percent of the cover present. Shrubland - Areas characterized by natural or semi-natural woody vegetation with aerial stems, generally less than 6 meters tall with individuals or clumps not touching to interlocking. Both evergreen and diciduous species of true shrubs, young trees, and trees or shrubs that are small or stunted because of environmental conditions are included. 51. Shrubland - Areas dominated by shrubs; shrub canopy accounts for 25-100 percent of the cover. Shrub cover is generally greater than 25 percent when tree cover is less than 25 percent. Shrub cover may be less than 25 percent in cases when the cover of other life forms (e.g. herbaceous or tree) is less than 25 percent and shrubs cover exceeds the cover of the other life forms. Non-natural Woody - Areas dominated by non-natural woody vegetation; non-natural woody vegetative canopy accounts for 25-100 percent of the cover. The non-natural woody classification is subject to the availability of sufficient ancillary data to differentiate non-natural woody vegetation from natural woody vegetation. 61. Orchards/Vineyards/Other - Orchards, vineyards, and other areas planted or maintained for the production of fruits, nuts, berries, or ornamentals. Herbaceous Upland - Upland areas characterized by natural or semi- natural herbaceous vegetation; herbaceous vegetation accounts for 75-100 percent of the cover. 71. Grasslands/Herbaceous - Areas dominated by upland grasses and forbs. In rare cases, herbaceous cover is less than 25 percent, but exceeds the combined cover of the woody species present. These areas are not subject to intensive management, but they are often utilized for grazing. Planted/Cultivated - Areas characterized by herbaceous vegetation That has been planted or is intensively managed for the production of food, feed, or fiber; or is maintained in developed settings for specific purposes. Herbaceous vegetation accounts for 75-100 percent of the cover. 81. Pasture/Hay - Areas of grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops. 82. Row Crops - Areas used for the production of crops, such as corn, soybeans, vegetables, tobacco, and cotton. 83. Small Grains - Areas used for the production of graminoid crops such as wheat, barley, oats, and rice 84. Fallow - Areas used for the production of crops that are temporarily barren or with sparse vegetative cover as a result of being tilled in a management practice that incorporates prescribed alternation between cropping and tillage. 85. Urban/Recreational Grasses - Vegetation (primarily grasses) planted in developed settings for recreation, erosion control, or aesthetic purposes. Examples include parks, lawns, golf courses, airport grasses, and industrial site grasses. Wetlands - Areas where the soil or substrate is periodically saturated with or covered with water as defined by Cowardin et al. 91. Woody Wetlands - Areas where forest or shrubland vegetation accounts for 25-100 percent of the cover and the soil or substrate is periodically saturated with or covered with water. 92. Emergent Herbaceous Wetlands - Areas where perennial herbaceous vegetation accounts for 75-100 percent of the cover and the soil or substrate is periodically saturated with or covered with water.

NLCD 1992 for Conterminous US by Class

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