Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

Raster-Based Geographic Data Sets—Logical Construction

In GIS a raster is a set of equally sized squares11 that cover a rectangular surface. Rasters are used in two basic ways in GIS:

• As rasters that store attribute information about the area covered by the square (such as elevation, or soil type), in which case individual squares are called cells.
• As images such as orthophotoquads, where the squares are picture elements (referred to as pixels), containing values that prescribe the intensities of visual colors (e.g., red, green, blue) as well as IR (infrared), UV (ultra violet), and thermal spectral elements.

Rasters (Grids)

The raster data model, which can be very useful in spatial analysis,12 comes in two flavors: those in which the cells contain integer numbers and those in which the cells contain floating-point numbers. In each case, each raster cell may contain a single number. If that number is in integer form, then the raster represents categorical or discrete data. Each different integer represents a type of object or a condition. If the numbers in cells are floating point—that is, they contain numbers that may have decimal fractions—then the raster may represent continuous data such as an elevation surface over the area of interest. Integer rasters usually13 have associated with them a value attribute table (VAT). Floating-point rasters usually do not have a VAT.

Each record in the VAT of an integer raster has a minimum of three fields: an ID field, a Value field, and ...