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Technical Article   FAQ:  What is the file structure of an Arc/INFO Grid?

Article ID: 30616
Software:  ArcInfo Workstation 9.0, 9.1, 9.2, 9.3 ArcGIS - ArcEditor 9.0, 9.1, 9.2, 9.3, 9.3.1, 10 ArcGIS - ArcInfo 9.0, 9.1, 9.2, 9.3, 9.3.1
Platforms: N/A

Question

What is the file structure of an Arc/INFO Grid?

Answer

Arc/INFO GRID format rasters are unique in their file structure. GRID rasters are stored in workspaces. A workspace contains one info subdirectory and a subdirectory for each GRID. Each GRID subdirectory contains several files that store geographic location and attribute data for the corresponding grid. The Info subdirectory contains several files which maintain certain information about the GRID and other datasets, such as coverages, in the workspace.



GRID directory

----------------

There are a number of files that can be stored in a GRID's subdirectory. Not all of them are necessary for the GRID to display. They can be stored in one of three storage types, INFO format, ASCII format and binary format.

dblbnd.adf

- The boundary file contains the minimum and maximum x,y coordinates for a grid

- INFO format



hdr.adf

- The header file contains information about the GRID's cell resolution, type (integer or floating point), compression, and tile information.

- binary format



log

- The log file contains a history about the creation of and alterations to a GRID.

- ASCII format



prj.adf

- The projection file stores the known map projection information of the GRID. It is optional.

- ASCII format



sta.adf

- The statistics (minimum, maximum, mean, and standard deviation) of the cell values.

- INFO format



vat.adf

- The value attribute table (VAT) stores the the attribute data associated with the zones of the grid. The vat.adf can only exist for integer GRIDs.

- INFO format



w001001.adf

- Stores the value of each cell in the first (base) tile of the GRID.

- If the GRID is floating point, the values are single precision.

- binary format



w001001x.adf

- Stores the index for the blocks in the tile.

- binary format



Note: If the grid has more than one tile, the value and index .adf file names will increment correspondingly. For example, the data files for tile 2 could be w001002.adf & w001002x.adf, tile 3 could be w001003.adf & w001003x.adf, and so on. The actual numbering of additional tiles is automatic and is based on their spatial relationship to the first tile.





INFO directory

--------------

The vat.adf, sta.adf, and dblbnd.adf files in a GRID sub directory have corresponding INFO files in the info directory, as .nit and .data pairs.



arcNNNN.dat (where N is any integer)

- Stores the relative path to the grid data file (e.g., "../soils/vat.adf")

- INFO format



arcNNNN.nit (note that for any particular grid, the NNNN values for .dat and .nit match)

- Stores file structure and field definition information of the GRID.

- INFO format



The info directory also contains also contains a data dictionary file called arc.dir.



Note: ArcGIS knows how to read and edit INFO format files. Do not try to edit the INFO files with other applications or remove with the operating system.





Other information:

--------------------

Some additional information on particular GRID files:



dblbnd.adf

- The minimum coordinates in the BND are for the lower-left corner of the lower-left cell in the GRID. The maximum coordinates are for the upper-right corner of the upper-right cell in the GRID.

- The coordinates are stored as double precision.



sta.adf

- the values in the statistics are floating point values.

- The values contained in this table should not be changed using INFO commands.

- Because NODATA represents an unknown value, NODATA is not used in calculating the statistics in the STA table.

- The standard deviation value -1 indicates that statistics have not been calculated for a grid.

- When a bilevel GRID (containing only 0 and 1 values) is created, the STA table will contain the value 0 for the mean and -1 for standard deviation.

- A standard deviation value of -2 indicates that the grid contains only NODATA cells.



vat.adf

- The vat.adf will always have a Value and a Count field.

- The Value field contains integer values that are used to distinguish the characteristics of one location from the other locations in a GRID. All cells that are assigned the same value contain the same characteristics and, therefore, belong to the same zone.

- The Count field stores the number of cells in a zone.

- The Value and Count fields should not be altered.

- Other attributes can be stored in additional fields.

- Never add new items before Value or Count.

- Cells containing NODATA are not represented in the attribute table.


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Created: 4/11/2006
Last Modified: 4/28/2011

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