a. Concept of Spatial Analysis
Spatial analysis is used to produce additional geographic information using existing information or to enhance the spatial structure or relationship between geographic information. Many techniques have been proposed, as follows.
b. Production of Additional Geographic Information
The following three techniques are very often used in GIS.
(1) Overlay technique (see Figure 13.5.1)
Various geographic data comprised of multiple layers are overlaid with logical operations including logical addition or logical multiplication. For example, a hazard risk area of soil erosion can be estimated by overlaying deforested and slope gradient maps in a mountainous area.
(2) Buffering technique (see Figure 13.5.2 )
Buffering is to find an area the within a certain distance from a given point or a line. For example noise polluted areas will be extracted by buffering an area within 30 meter distance from a trunk road.
(3) Volonoi tessellation
An area may be divided in a group of "influential areas" termed Volonoi tessellation, that can be formed by bisectors between spatially distributed points. For example, a school zone can be drawn by Volonoi tessellation between differently located schools.
c. Statistical Analysis for Spatial Structure
Spatial auto-correlation is one of the statistical techniques to find the spatial structure of geographic information. Spatial auto-correlation is a correlation factor between two differently located events. High accuracy spatial interpolation can be executed with a lower density of samples in the case of high spatial auto-correlation.
d. Combined Technique
Figure 13.5.3 shows an example of a combined technique using remote sensing, buffering and overlay. In this example, the land use change ratio is tabulated with respect to accessibility to a railway station and land use zoning.
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