United States Environmental Protection Agency

Office of Research and Development

National Exposure Research Laboratory
Research Abstract

Government Performance Results Act (GPRA) Goal 4
Annual Performance Measure 277

Significant Research Findings:

Accuracy assessment of the 1992 multi-resolution land
characteristics (MRLC) national land cover data (NLCD)

Scientific	Many air and water-quality models use land cover as a primary data source.

Problem and	Accuracy of land-cover data are therefore important for understanding model

Policy Issues	results. A statistically rigorous sampling design was used to collect reference data

to document the thematic accuracy of the 1992 NLCD, the most widely used land-
cover data across EPA.

Research	The main objective of this task was to document the thematic accuracy of the 1992

Approach	NLCD. We used a two-stage cluster sampling design to collect 18,000 reference

samples across the conterminous United States. The land-cover labels for the
photo-based reference samples were compared to the satellite derived map land-
cover labels to determine the thematic accuracy.

Results and	Thematic accuracy results show that 1992 NLCD meets nominally established

Impact	standards (85%) for 8 of 10 EPA Regions and Anderson Level I. Level I thematic

accuracy was only 74% for the south-central region (Region 6) and 70% for the
mid-Atlantic region (Region 3). The lower overall Level I accuracy for Region 6
was due primarily to confusion between pasture and grassland, and the lower
overall Level I accuracy for the mid-Atlantic was due primarily to confusion
between forest and agriculture. Level II and I class-specific users accuracies and
associated standard errors are reported in Stehman et al. (2002) and Wickham et
al. (2004), and these results are also reported at htto://www .epa.gov/mrlc . The
results also demonstrate that land-cover heterogeneity (i.e., many different classes
within a small area of the map) and ambiguity in reference label assignment were
the primary sources of error. The subset of samples for which no other land-
cover classes were within 30 meters showed significantly higher accuracy.
Most of the classification error is at the edges between land-cover classes.

Research
Collaboration and
Research
Products

The research was done in collaboration with the U.S. Geological Survey and State
University of New York (SUNY-ESF). The published research and results are
listed below, and reported on the internet at http://www.epa.gov/mrlc/.

Wickham JD, Stehman SV, Smith JH, and Yang L. 2004. Thematic accuracy of 1992 National
Land-Cover Data for the western United States. Remote Sensing of Environment 91:452-468.

Wickham JD, Stehman SV, Smith JH, Wade TG, and Yang L. 2004. A priori evaluation of two-
stage cluster sampling for accuracy of large-scale land-cover maps. International Journal of Remote


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Sensing 25:1235-1252.

Stehman SV, Wickham JD, Smith JH, and Yang L. 2003. Thematic accuracy of the 1992 National
Land-Cover Data (NLCD) for the eastern United States: statistical methodology and regional
results. Remote Sensing of Environment 86:500-516.

Smith JH, Stehman SV, Wickham ID, and Yang L. 2003. Effects of landscape characteristics on
land-cover class accuracy. Remote Sensing of Environment 84:342-349.

Smith JH, Wickham JD, Stehman SV, and Yang L. 2002. Impacts of Patch Size and Land Cover
Heterogeneity on Thematic Image Classification Accuracy. Photogrammetric Engineering and
Remote Sensing 68:65-70.

Future Research Research on change detection accuracy assessment is underway.

James D. Wickham

U.S. EPA, Office of Research and Development
National Exposure Research Laboratory
www. epa.gov/mrlc/accuracy .html
Phone: 919/541-3077
E-mail: wickham.james@epa.gov

Funding for this project was through the U.S. EPA's Office of Research and
Development, National Exposure Research Laboratory, and the work was
conducted by the Environmental Science Division. Collaborators were funded
through IAGs DW14937937-01, DW14938034-01, DW14939340-01,
DW14939330-01, and DW14939339-01, and contracts 9V1031NAEE, 68-C5-
0065, 68-D-00-267, and 68-W60023.

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