In raster overlay, the pixel or grid cell values in each map are combined using arithmetic and Boolean operators to produce a new value in the composite map. The maps can be treated as arithmetical variables and perform complex algebraic functions. The method is often described as map algebra. The raster GIS provides the ability to perform map layers mathematically. This is particularly important for the modelling in which various maps are combined using various mathematical functions. Conditional operators are the basic mathematical functions that are supported in GIS.
Conditional operators were already used in the examples given above. The all evaluate whether a certain condition has been met.
= eq 'equal' operator
<> ne 'non-equal' operator
< lt 'less than' operator
<= le 'less than or equal' operator
> gt 'greater than' operator
>= ge 'greater than or equal' operator
Many systems now can handle both vector and raster data. The vector maps can be easily draped on to the raster maps.
Using these operations, the characteristics of an area surrounding in a specified location are evaluated. This kind of analysis is called proximity analysis and is used whenever analysis is required to identify surrounding geographic features. The buffer operation will generate polygon feature types irrespective of geographic features and delineates spatial proximity. For example, what are the effects on urban areas if the road is expanded by a hundred meters to delineate a five-kilometer buffer zone around the national park to protect it from grazing.
Digital Terrain Model
The object of Digital Terrain analysis is to represent a surface and its properties accurately. This is normally achieved by creating a digital terrain model, often known as DTM, formed by sampling the surface. A digital terrain model can be viewed in two different ways:
- as an isoline map,
- as an isometric model.
Isolines join points of equal value on a surface. The shading defines bands, including all heights, between the isolines.
Isometric models can be shown in three-dimensional models. These models show the terrain in perspective so that the apparent height is proportional to the value of the point. Visualisation techniques are used to project the model from the given eyepoint.
Spatial Analysis - a Process
Spatial Analysis a Process