The USGS Land Cover Institute (LCI)
NLCD 1992 Frequently Asked Questions (FAQ)
What was the motivation for the creation of the National Land Cover Data Set?
Many federal agencies, including the U.S. Geological Survey, the U.S. Environmental Protection Agency, the U.S. Forest Service, the National Oceanic and Atmospheric Administration, state goverments, and various environmental groups need up-to-date intermediate scale land cover data. The most recent intermediate scale land cover data set generated for the conterminous United States was developed by the USGS in the 1970's. Although this data set is probably still adequate for some applications, many land cover changes have occurred since the data set was compiled. The main objective of this project is to generate a relatively current, consistent, seamless, and accurate land cover data set for the conterminous United States.
Why is land cover data needed, i.e., what are the applications?
Potential uses of land cover data are many and varied, and include assessing ecosystem status and health, modeling nutrient and pesticide runoff, understanding spatial patterns of biodiversity, land use planning, deriving landscape pattern metrics, and developing land management policy.
Who is involved in the project?
Six federal environmental monitoring programs have formed a partnership (known as the Multi-Resolution Land Characterization Consortium) with the EROS Data Center (U.S. Geological Survey) to facilitate the development of the land cover data base for the United States. The programs and their major constituents include the following:
USGS Gap Anaylsis Program (GAP)
National Water Quality Assessment Program (NAWQA) -- USGS
Coastal Change Analysis Program (C-CAP) -- National Oceanic and Atmospheric Administration (NOAA)
North American Landscape Characterization (NALC) -- USGS and EPA
Remote Sensing Applications Center (RSAC) -- U.S. Forest Service
How do I acquire the data?
All NLCD data products are available for download at no charge from the NLCD MRLC web site www.mrlc.govReturn to Categories
Data Format/Importing into an application
In what format are the data provided?
National Land Cover Data distributed on CD-ROM is in GeoTIFF format. Software to view the Geo-TIFF (DLGV32) is also provided on the CD. The data that are directly downloaded are generic binary 'flat files' and have been compressed with the "gzip" utility to shorten download times.
How are the compressed data uncompressed?
To speed download time, the data available for download are compressed using the "gzip" utility. The "gzip" utiliity is a standard file compression utility that is available on a variety of hardware platforms (from Windows to Unix machines) and is likely available to the vast majority of users. PC users can use WinZip. Unix users simply type:
How do I view/use the data with my image processing/GIS application?
The Geo-TIFF on the CD can be IMPORTED into ERDAS. The "IMAGEGRID" command can be used to ingest the Geo-TIFF into ARC. The downloaded data are provided in a generic image format and should be easily imported into most geographic information system and image processing packages. Due to the large variety of software packages available, it isn't feasible to include instructions on importing the data to all possible software packages. However, information on importing the data into two commonly used software packages may be viewed by clicking on the following:
For other software packages, consult your software manual for instructions on importing generic raster image data.
NOTE: This is not an endorsement of ArcInfo or ERDAS software.
The data are not in the projection I need. Can I get the data in another projection?
At this time, the data are only provided in an Albers Conical Equal Area projection. Most image processing/GIS packages have the ability to reproject raster data; consult your software manual for reprojection instructions.Return to Categories
Land cover data questions/concerns
From what timeframe is the source Landsat TM data used to build the classification?
Attempts were made to acquire Landsat TM data for the years near 1992. Due to data quality issues or a lack of available data, occasionally scenes were used that were 3 or 4 years prior to the target acquisition date.
I've noticed a problem in the classification for the state I downloaded. Should I inform NLCD personnel?
Definitely! Feedback from users is a critical element in the modification of our preliminary land cover products. Should you notice any problems in the land cover data set you have downloaded, contact members of the NLCD project team by e-mailing email@example.com.
How is classification accuracy assessed?
The NLCD are assessed for accuracy based upon the Federal Regions (i.e. the reported accuracy is for the Region as a whole, rather than for individual states). Accuracy assessments follow a revision cycle that incorporates feedback from MRLC Consortium partners and affiliated users. Private sector vendors under contract to the USEPA conduct the accuracy assessments. A protocol has been established by the USGS and USEPA 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 accuracies, and omission and commission error probabilities. For a detailed discussion see:
Zhu, Z., Yang, L., Y., Stehman, S.V., and Czaplewski, R.L, Accuracy Assessment for the U.S. Geological Survey Regional Land-Cover Mapping Program: New York and New Jersey Region, Photogrammetric Engineering and Remote Sensing 66, No. 12: pg 1425-1435.
The classification scheme in the README files shows 21 land cover classes, yet the data I downloaded shows only 16 distinct classes. Why the difference?
Some classes are simply not found in certain states (there is no perennial ice/snow in Florida!). However, the exclusion of some classes is simply a function of what is possible with the classification of Landsat thematic mapper data alone. The "planted/cultivated woody", for example, refers to features such as an orchard or a vineyard. Spectrally, these are extremely difficult to differentiate from other land cover classes (an orchard verses a forest, for example). We include them in our classification if ancillary data can be found which helps to identify their locations. If no such data are available, the class is not included in the classification. You may wish to consult the "Caveats and Concerns" in the "README" file for the state you're interested in for more information.Return to Categories
I need information that's not included on these web pages. What can I do?
Members of the National Land Cover Data (NLCD) project team may be contacted by e-mailing firstname.lastname@example.org. In addition, you can check out a pair of publications on the project:
Vogelmann, J., Sohl, T., and Howard, S., 1998. Regional Characterization
of Land Cover Using Multiple
Vogelmann, J.E., Sohl, T.L., Campbell, P.V., and Shaw, D.M., 1998. Regional
land cover chacterization
Vogelmann, J.E., S.M. Howard, L. Yang, C.R. Larson, B.K. Wylie, N. Van Driel, 2001. Completion of the 1990s National Land Cover Data Set for the Conterminous United States from Landsat Thematic Mapper Data and Ancillary Data Sources, Photogrammetric Engineering and Remote Sensing, 67:650-652.