One of the weak points in raster model is the difficulty in network and spatial analysis as compared with vector model.
For example, though a line is easily identified as a group of pixels which form the line, the sequence of connecting pixels as a chain would be a little difficult in tracing. In case of polygons in raster model, each polygon is easily indentified but the boundary and the node (when at least more than three polygons intersect) should be traced or detected.
a. Flow directions
A line with directions can be represented by four directions called as the Rook's move in the chess game or eight directions called as the Queenís move, as shown in Figure 2.7 (a) and (b).
Figure 2.7 (c) shows an example of flow directions in the Queen's move. Water flow, links of a network, roads etc. can be represented by the flow directions (or called Freeman chain code).
b. Topological Features of Raster Data
Boundary is defined as 2 x 2 pixel window that has two different classes as shown in Figure 2.8 (a).
If a window is traced in the direction shown in Figure 2.8 (a), the boundary can be indentified.
c. Node
A node in polygon model can be defined as a 2 x 2 window that has more than three different classes as shown in
Figure 2.8 (b).
Figure 2.8 (c) and (d) show an example of identification of pixels on boundary and node.