See Also:

http://geo-nsdi.er.usgs.gov/metadata/other/epa/giras-lulc/sdd/metadata.html

http://water.usgs.gov/GIS/metadata/usgswrd/huc250.html

USGS GIRAS FAQ

1:250,000-scale Land Use and Land Cover (LULC)



The data files adhere to the follow naming convention:

land_use = Land Use and Land Cover (GIRAS format) political = Political Units (GIRAS format) hydro = Hydrologic Units (GIRAS format) census = Census County Subdivision (GIRAS format) federal = Federal Land (GIRAS format) state = State Land (GIRAS format) grid_cell = Composite Theme Grid format (all layers)

FILE COMPRESSION The files have been compressed with the GNU "gzip" utility.

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 do NOT contain record delimiters. Once you down load the files, you can add delimiters using the following UNIX command:

dd if=inputfilename of=outputfilename ibs=8000 cbs=80 conv=unblock

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.



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".

Classification Codes 
Classification Codes-first and second level categories 

1 Urban or Built-Up Land 
11 Residential 
12 Commercial Services 
13 Industrial 
14 Transportation, Communications 
15 Industrial and Commercial 
16 Mixed Urban or Built-Up Land 
17 Other Urban or Built-Up Land 
2 Agricultural Land 
21 Cropland and Pasture 
22 Orchards, Groves, Vineyards, Nurseries 
23 Confined Feeding Operations 
24 Other Agricultural Land 
3 Rangeland 
31 Herbaceous Rangeland 
32 Shrub and Brush Rangeland 
33 Mixed Rangeland 
4 Forest Land 
41 Deciduous Forest Land 
42 Evergreen Forest Land 
43 Mixed Forest Land 
5 Water 
51 Streams and Canals 
52 Lakes 
53 Reservoirs 
54 Bays and Estuaries 
6 Wetland 
61 Forested Wetlands 
62 Nonforested Wetlands 
7 Barren Land 
71 Dry Salt Flats 
72 Beaches 
73 Sandy Areas Other than Beaches 
74 Bare Exposed Rock 
75 Strip Mines, Quarries, and Gravel Pits 
76 Transitional Areas 
77 Mixed Barren Land 
8 Tundra 
81 Shrub and Brush Tundra 
82 Herbaceous Tundra 
83 Bare Ground 
84 Wet Tundra 
85 Mixed Tundra 
9 Perennial Snow and Ice 
91 Perennial Snowfields 
92 Glaciers