LULC (Land Use Land Cover) data for locations in the United States has been provided by the USGS (U.S. Geological Survey) over the years using several different formats. Legacy LULC data sets are often found that use two different formats for LULC data for the same region: the GIRAS (Geographical Information Retrieval and Analysis System) vector format, and CTG (Composite Theme Grid) raster format. CTG format is also known as Grid Cell format since it is often published in a file called grid_cell. LULC data in these formats can usually be found for each of the 1:100K and 1:250K regions traditionally served by USGS.
Files in either CTG or GIRAS format do not use three-letter extensions. For example, downloading and then decompressing any compressed files for the San Francisco 100K LULC data set from USGS will result in five files, as seen in Windows File Explorer:
One of the files, the grid_cell file, is raster data in CTG format. The other four files contain vector data in GIRAS format. The five files provide the following information:
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)
It is not always easy to remember that the grid_cell file uses a different format, so Manifold users often will rename the grid_cell file to grid_cell.ctg to indicate it is a raster file in CTG format, a different format used than the other four files.
The grid_cell.ctg file can then be imported into Manifold using the CTG dataport.
To import from LULC CTG format:
Choose File-Import from the main menu.
In the Import dialog browse to the location of the file.
Chose the . Files (CTG LULC) (*.*) option.
Double-click on the the grid_cell file or grid_cell.ctg file.
A table and four images will be created.
The four images each use a different tile column in the grid_cell Tiles table.
For a step-by-step example of importing and using Style with a CTG file, see the Example: Import CTG Grid Cell File and Style topic.
Projections - CTG grid cell format stores projection information so normally the imported images will be in the correct coordinate system.
Classification Codes - The value for each pixel in the grid_cell LULC is a classification code that can be matched to a palette color to provide a more understandable display, as illustrated in the Example: Import CTG Grid Cell File and Style topic. USGS defines the following codes:
1 Urban or Built-Up Land
12 Commercial Services
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
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
51 Streams and Canals
54 Bays and Estuaries
61 Forested Wetlands
62 Non-forested Wetlands
7 Barren Land
71 Dry Salt Flats
73 Sandy Areas Other than Beaches
74 Bare Exposed Rock
75 Strip Mines, Quarries, and Gravel Pits
76 Transitional Areas
77 Mixed Barren Land
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
Oceans may be coded with a value of 2000000102 or 0.
Assign Initial Coordinate System
GIRAS, LULC Vector Format
Example: Import CTG Grid Cell File and Style - We import a CTG grid cell file containing raster data showing land use and land cover and then we use Style to provide a more understandable display.