| # GRASS Help System

## QUESTIONS OF SCALE, RESOLUTION AND ACCURACY

In raster data format, data are stored on a grid containing rectangular cells of uniform size. The cell is the smallest unit in which information can be stored. Cell dimensions are defined by the user.

Each cell within the grid can be assigned a data value and a category label describing the type of data found there. Because each cell covers an area rather than a single point, this data value may be a generalization of all the actual data values found within this area on the earth. Whether the data values assigned to each cell represent a generalization, or represent all of the actual data found within the cell, will depend on the cell's dimensions and the specific nature of the data. The user should define cell dimensions carefully.

Grid cell size is linked to map scale and resolution. Smaller grid cell size allows more data to be stored for a given geographic area. Therefore, smaller grid cells are associated with a larger scale and a finer resolution than are larger grid cells. The cell resolution represents the size (area) of the piece of the earth to be represented by one cell in a raster map layer.

For a graphic example illustrating the impact of different cell resolutions, see Raster Map Layer Resolution: An Example.

Each cell is also associated with a specific geographic x,y coordinate location. In GRASS, the user specifies cell dimensions in the x (east-west) and y (north-south) directions, for each map. It is permissible to make the east-west resolution different from the north-south resolution.

Once the resolution of a new raster map layer is set, the geographic information available in that map layer is set. The user may subsequently use the GRASS g.region command to change the working resolution of a raster map layer, but the original resolution of the map remains intact and available to the user regardless of any resampling performed as a result of changes in the cell resolution designation in the current geographic region.

Increasing the resolution of an existing raster map layer will only subdivide the information from the existing cells. It is important to understand that increasing the cell resolution of an existing raster map layer cannot actually access more data than were originally present.

For further information read: "Cartographic Issues in the Development of a Digital GRASS Database".

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Automatically created on: Sat Oct 15 07:07:26 2005