Licensing A project of Ecotrust
This directory contains the 1:250,000-scale Land Use and Land Cover (LULC) Data.
*** If you have any questions or suggestions, please contact Customer Services at custserv@edcserver1.cr.usgs.gov ***
DIRECTORY STRUCTURE : The directory structure for the 1:250,000-scale LULC FTP account is defined as follows: A-Z/mapname where:
The data files, which are located under the map name subdirectory, adhere to the follow naming convention:
FILE COMPRESSION: The files have been compressed with the GNU "gzip" utility. If you do not have access to gzip, the FTP server will uncompress the file as you retrieve it. To do this, simply leave off the ".gz" extension when retrieving the file (NOTE: This option is not available through MOSAIC). For example, to retrieve the file "census.gz" without compression just use "get census". Note that the uncompressed files are typically five times larger than the compressed versions and will take five times longer to transmit. The gzip program is available via anonymous FTP at the following sites: prep.ai.mit.edu:/pub/gnu wuarchive.wustl.edu:/systems/gnu The data files are stored on a robotic mass-storage device so there may be a short (~10 seconds) delay while the media is retrieved.
DATA CONTENT: The set of Land Use and Land cover and associated maps consists of Land Cover, political units, hydrologic units, census county subdivisions, Federal land ownership, and State land ownership. The Land Use and Land Cover map is compiled to portray the Level II categories of the Land Use and Land Cover classification system documented by Anderson and others (1976). The Level II categories of this Land Use and Land Cover classification system provide the user with a basic framework to which third- and fourth-level categories may be added. The associated maps portray either natural or administrative information. They provide the user with the opportunity to utilize the Land Use and Land Cover maps and data, either individually or collectively, to produce graphic or tabular data for the areas portrayed on the associated maps. This mapping system is constructed in such a way that the Land Use and Land Cover data can be related to other resource fields such as soils, geology, hydrology, and demography.
DATA FORMAT: The LULC data are available in two formats: GIRAS and CTG. GIRAS Format: The geographic information retrieval and analysis system (GIRAS) format involves a standard character fixed length record (usually ASCII-coded, 80-character logical record). Each record may consist of 1 to 16 data- element fields, and each data element may be one of three different types: 1) 16-bit binary integer: 2) 32-bit binary integer or 3) a string of text characters. The GIRAS file structure is comprised of a map header, section header, arc records subfile, coordinate subfile, polygon records subfile, file of arcs by polygon (FAP) subfile, text subfile, and an associated data subfile. The LULC GIRAS files located in the FTP account do NOT contain record delimiters. Once you down load the files, you can add delimiters using the following UNIX command:
CTG Format: The Composite Theme Grid (CTG) format involves representing data in raster or grid cell form for a given quadrangle. The CTG files are sequential and consist of fixed length records (except for header files) with one grid cell for each logical record. The grid cells are actually a regular point sample of the quad where the center point of each cell is 200 meters apart from other center points in adjacent cells. The cells are mapped to the UTM projection and oriented in the north-south, east-west directions. For a complete explanation of the 1:250,000-scale LULC data see the USGS National Mapping Program, Technical Instructions, Data Users Guide 4, "Land Use and Land Cover Digital Data from 1:250,000- and 1:100,000-scale Maps". This booklet can be ordered, for a small fee, from the Earth Science Information Center at the following address: Earth Science Information Center U.S. Geological Survey 507 National Center Reston, VA 22092 Tel: 783-648-6045 or 800-USA-MAPS Fax: 783-648-5548 Technical Instructions can also be obtained from the following anonymous FTP account: ftp://map.usgs.gov/pub/ti/LULC/lulcguide The Global Land Information System (GLIS) also provides on-line data set documentation. You can reach the GLIS system from the following URL: http://edcwww.cr.usgs.gov/glis/glis.html
IMPORTANT NOTE: Coordinates in the LULC files are based on the UTM projection, but are NOT true UTM coordinates--they result from a linear transformation that scales them into a 16 bit coordinate space. As a result of this transformation, GIRAS quads will substantially overlap when plotted together. The coordinate and overlap problems result from a decision to store GIRAS coordinates in 16-bit integer fields. 16-bit integer fields store values ranging in magnitude from -32,768 to 32,767. Actual UTM coordinates can exceed 4,000,000 meters, greatly exceeding the useable 16-bit range of numbers. Large coordinate values can be reduced in magnitude by establishing a coordinate near them as a "local zero". The master coordinate value of this shifted zero point can then be subtracted from all of the points of interest, resulting in a translation of the X and Y axes to the "local zero" and providing smaller magnitude coordinates. Even with this reduction in magnitude, UTM coordinates for a 1 by 2 degree 1:250,000 quad may exceed a 190,000 meter range. A further reduction in magnitude can be gained by dividing the translated coordinates by 10. This is how GIRAS coordinates were calculated. The "local zero" was chosen to be the closest 100,000 meter grid intersection that falls both west and south of all the map control points. Map control points are usually the corner points of the map quad. To reconstitute UTM coordinates from the GIRAS coordinates provided, multiply the local coordinate value by 10 and add the UTM coordinate of the "local zero" 100,000 meter grid intersection. Note that the division by 10 quantized the data, causing small overshoots and undershoots along quad boundaries when UTM coordinates are recalculated. A complete explanation and processing information can be found in the Technical Instructions, Data Users Guide 4, "Land Use and Land Cover Digital Data from 1:250,000- and 1:100,000-scale Maps". As an alternative to further processing, you may retrieve converted GIRAS files, in an ARC Export format, from EPA at the following FTP account: ftp.epa.gov:/pub/EPAGIRAS You can also obtain the EPAGIRAS data from the following URL: http://www.epa.gov/