Overlay of raster data with more than two layers is rather easier as compared with overlay of vector data, because it does not include any topological operation but only pixel by pixel operations.
Generally there are two methods of raster-based overlay.
Weighting point method: basically two layers with the values of P1 and P2 respectively are overlaid with the weight of w1 and w2 respectively as follows.
The weighting point method is only available when the attributes have numerical values which can be operated arithmetically.
Ranking method: at first the attributes of the two layers are categorized into five ranks as excellent (5), better (4), good (3), poor (2), and bad (1) before a specific purpose of overlay. Then the two different layers of A and B are overlaid by following one of the three ranking tables as shown in Table 4.1.
Minimum Ranking
Lower rank is taken as the new rank of the overlaid pixel as the safety rule.
Multiplication Ranking
Two ranks are multiplied because of more influential effect rather than additional effect.
Selective Ranking
Experts can set up combined ranks depending on professional experience.
For practical purposes, a model of overlay with many layers and the hierarchical structure should be built by users as shown in Figure 4.6.