GEOG 160
Mapping Our Changing World

7.7 Case Study: Using Landsat for Land Cover Classification for NLCD


The USGS developed one of the first land use/land cover classification systems designed specifically for use with remotely sensed imagery. The Anderson Land Use/Land Cover Classification system, named for the former Chief Geographer of the USGS who led the team that developed the system, consists of nine land cover categories (urban or built-up; agricultural; range; forest; water; wetland; barren; tundra; and perennial snow and ice), and 37 subcategories (for example, varieties of agricultural land include cropland and pasture; orchards, groves, vineyards, nurseries, and ornamental horticulture; confined feeding operations; and other agricultural land). Image analysts at the U. S. Geological Survey created the USGS Land Use and Land Cover (LULC) data by manually outlining and coding areas on air photos that appeared to have homogeneous land cover that corresponded to one of the Anderson classes.

The LULC data were compiled for use at 1:250,000 and 1:100,000 scales. Analysts drew outlines of land cover polygons onto vertical aerial photographs. Later, the outlines were transferred to transparent film georegistered with small-scale topographic base maps. The small map scales kept the task from taking too long and costing too much, but also forced analysts to generalize the land cover polygons quite a lot. The smallest man-made features encoded in the LULC data are four hectares (ten acres) in size, and at least 200 meters (660 feet) wide at their narrowest point. The smallest non-man-made features are sixteen hectares (40 acres) in size, with a minimum width of 400 meters (1320 feet). Smaller features were aggregated into larger ones. After the land cover polygons were drawn onto paper and georegistered with topographic base maps, they were digitized as vector features, and attributed with land cover codes. A rasterized version of the LULC data was produced later.

The successor to LULC is the USGS's National Land Cover Data (NLCD). Unlike LULC, which originated as a vector data set in which the smallest features are about ten acres in size, NLCD is a raster data set with a spatial resolution of 30 meters (i.e., pixels represent about 900 square meters on the ground) derived from Landsat TM imagery. The steps involved in producing the NLCD include preprocessing, classification, and accuracy assessment, each of which is described briefly below.

7.7.1 Preprocessing

The first version of NLCD--NLCD 92--was produced for subsets of ten federal regions that make up the conterminous United States. The primary source data were bands 3, 4, 5, and 7 (visible red, near-infrared, mid-infrared, and thermal infrared) of cloud-free Landsat TM scenes acquired during the spring and fall (when trees are mostly bare of leaves) of 1992. Selected scenes were geometrically and radiometrically corrected, then combined into sub-regional mosaics comprised of no more than 18 scenes. Mosaics were then projected to the same Albers Conic Equal Area projection (with standard parallels at 29.5° and 45.5° North Latitude, and central meridian at 96° West Longitude) based upon the NAD83 horizontal datum.

7.7.2 Image Classification

An unsupervised classification algorithm was applied to the preprocessed mosaics to generate 100 spectrally distinct pixel clusters. Using aerial photographs and other references, image analysts at USGS then assigned each cluster to one of the classes in a modified version of the Anderson classification scheme. Considerable interpretation was required, since not all functional classes have unique spectral response patterns.

Table 7.4: Modified Anderson Land Use/Land Cover Classification used for the USGS National Land Cover Dataset.
Level I Classes Level II 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
Forested Upland 41 Deciduous Forest
42 Evergreen Forest
43 Mixed Forest
Shrubland 51 Shrubland
Non-Natural Woody 61 Orchards/Vineyards/Other
Herbaceous Upland Natural/Semi-natural Vegetation 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

Table credit: USGS.

7.7.3 Accuracy Assessment

The USGS hired private sector vendors to assess the classification accuracy of the NLCD 92 by checking randomly sampled pixels against manually interpreted aerial photographs. Results from the first four completed regions suggested that the likelihood that a given pixel is correctly classified ranges from only 38 to 62 percent. Much of the classification error was found to occur among the Level II classes that make up the various Level I classes, and some classes were much more error-prone than others. USGS encourages users to aggregate the data into 3 x 3 or 5 x 5 pixel blocks (in other words, to decrease spatial resolution from 30 meters to 90 or 150 meters), or to aggregate the 21 Level II classes into the nine Level I classes.

An extract from NLCD 92. More in surrounding text.
Figure 7.27. An extract from NLCD 92 that corresponds to the same portion of the Bushkill, PA quadrangle mapped in other USGS data files provided with earlier chapters. The data viewer is ESRI's ArcExplorer version 2.
Credit: USGS.
Map legend for the National Land Cover Dataset. Includes columns for a Color Key, RGB Value, and Class Number and Name
Figure 7.28. Map legend for the National Land Cover Dataset.
Click for a text description of Figure 7.28

National Land Cover Dataset Classification System Legend:

Color Key RGB Value Class Number and Name
Blue 0, 0, 255 11 Open Water
White 255, 255, 255 12 Perenniallce/Snow
Light Orange 255, 204, 0 21 Low Intensity Residential
Orange 255, 153, 0 22 High Intensity Residential
Red 255, 0, 0 23 Commercial/Industrial/Transportation
Eggshell White 229, 229, 204 31 Bare Rock/Sand/Clay
Brown 128, 77, 51 32 Quarries/Strip Mines/Gravel Pits
Neon Pink 255, 0, 255 33 Transitional
Green 0, 178, 0 41 Deciduous Forest
Dark Green 0, 102, 0 42 Evergreen Forest
Teal 0, 178, 178 43 Mixed Forest
Olive Green 178, 178, 0 51 Shurbland
Purple 153, 25, 229 61 Orchards/Vineyards
Tan 229, 204, 153 71 Grassland/Herbaceous
Yellow 255, 255, 0 81 Pasture/Hay
Light Pink 255, 179, 204 82 Row Crops
Pink 204, 77, 128 83 Small Grains
Gray 178, 178, 178 84 Fallow
Neon Green 128, 255, 0 85 Urban/Recreational Grasses
Seafoam 128, 255, 204 91 Woody Wetlands
Neon Teal 0, 255, 255 92 Emergent Herbacious Wetlands
Credit: USGS.