Raster

Cell (Pixel) Dimensions

Raster data have a number of characteristics that determine how GIS operations may be performed. A raster file has a specified number of rows and columns of data that represent the values of a particular phenomenon over a specific area on the earth's surface. If each cell has the same spatial extent in x and y, the area represented by each cell is consistent throughout the raster. Areas within the image may be calculated easily by multiplying the area covered by each cell by the number of cells. This is the most widely used pixel shape in remote sensing and GIS.

However, raster data may have other shapes. Raster data sets are often created using collection points separated by distances measured in terms of degrees, minutes, or seconds of longitude and latitude. In this case, the area of all pixels is not the same, changing as one moves north and south through the data set. Thus, if the raster covers a significant area on the earth's surface, the ground area of a cell is less the farther it is away from the equator because of convergence of lines of longitude at the poles.

Another shape for cells in a raster data set is hexagonal, which is used for numerical modeling to ensure uniform spacing between points in diagonal as well as rectangular directions. Other shapes (triangles, irregular rectangles, etc.) can be used, each with their own characteristics in regard to spatial data functions, such as angular, linear, and area measurement.

The geographic coordinates of a raster data set are normally stored in an ancillary file or in a header record within the raster file. Since a raster is (usually) a regular grid, only the top-left geographic coordinates are needed to compute the geographic position of any cell or pixel within the raster file. A raster file may contain more than one spatial variable (land cover, roads, slope, etc.); however, the cell size of each layer represented within a single raster file must [Page 362]be the same. This same architecture supports the ability of a raster file to contain geocorrected multiband images. Each spectral band of the multispectral image may be treated as a separate variable.

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