CTG, LULC Grid Cell

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.  

LULC Grid Cell Files

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:  



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:


  1. Choose File-Import from the main menu.

  2. In the Import dialog browse to the location of the file.

  3. Chose the . Files (CTG LULC) (*.*) option.

  4. Double-click on the the grid_cell file or grid_cell.ctg file.

  5. 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

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 Non-forested 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


Oceans may be coded with a value of 2000000102 or 0.


See Also

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.