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A geographical information system (GIS) is a computer-based system that handles various forms of data dealing with advanced mapping. It can, among other things, gather, store and analyze data for use in a variety of fields, such as urban planning and archeology. A raster-based GIS, or simply raster GIS, uses a combination of cartography and imagery data to give a complete representation of the area being studied. A grid is used to represent imagery data and calculations can be performed with the data to reveal relationships between attributes of the target area. Raster GIS is commonly used to classify vegetation, land use, and urban change over a particular area.
Raster GIS has the benefit of being able to collect data for all geographic features, images, and surfaces. Such systems are good for representing data that can be measured at every point, such as temperature and elevation. A raster cell records information about the interior of the cell and boundaries are created by adjacent cells.
A raster-based GIS represents datasets by dividing a map into equally sized cells aligned in a grid. The cells can be assigned a value used to represent some characteristic about the location, and all of the cells of one data set make up a layer, or raster. The layer-and-cell format of the raster GIS dataset allows multiple pieces of data to be positioned in the same place. When layers are stacked, relationships are formed between the different datasets that occupy the same cell. The ability to compare the relationships within different cells of a given area is what makes raster GIS such a useful tool.
A GIS technician can use overlaid raster GIS layers to create new layers using algebraic equations. The algebraic equations, or map algebra, allow new layers to be created by using the information of the GIS system to eliminate areas of land that do not meet the user-defined criteria. The ability to stack layers of information makes raster GIS a good tool for determining sites that meets specific requirements.
Advances in aircraft and satellite technology have allowed large improvements in the ability and ease of using remote sensing to gather raster GIS data. Remote sensing instruments are fixed to aircraft and satellites and, from there, gather different types of imagery data. The remote sensing instruments can detect and record both visible data and any measurable part of the electromagnetic spectrum.
The drawback of this system is that linear features, such as rivers, are hard to represent accurately. The size of the cell determines how accurate the boundaries and features are portrayed. A smaller cell allows for more precise boundaries but requires more data processing.