# GRASS Help System

What Are Raster Data?

Raster data format uses the cell as its fundamental unit of information. Data are stored on a grid containing rectangular cells of uniform size. Cell dimensions denote data resolution. Each cell within the grid can be assigned a data value. Smaller grid cell size therefore allows more data to be stored for a given geographic area (i.e., it can present data at a larger scale).

Each cell is assigned a value (aka, category value) and a label (aka, category label) describing the type of data found there. Each cell is also associated with a specific geographic x,y coordinate location.

Each GRASS raster data file describes a specific thematic map in terms of its geographic coordinates. Supporting files describe the category values, category labels, map title, history, etc., corresponding to this raster data file. All of the files relating to a given raster data file are collectively termed a raster map layer.

NOTE: "Cell map" was the term used to denote maps in raster format in previous versions of GRASS. A "cell map" is now referred to as a "raster map" in GRASS 5.0 documentation.

Raster data files are stored under the "cell" directory under each mapset. For more information on the location and content of raster data and supporting files, see the topic "GRASS Database Directory Structure," accessible from this area's main menu.

Although users can input, display, and output maps in GRASS vector and sites file formats, most GRASS analysis programs function on raster data. If the user wishes to analyze data, he generally must convert his data to raster data format. Imagery data are already in raster data format.

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Automatically created on: Sat Jan 8 06:50:25 2005