Several records or files are hierachically related with each other. For example, an organization has several departments, each of which has attributes such as name of director, number of staffs, annual products etc.
Each department has several divisions with attributes of name of manager, number of staffs, annual products etc.
Then each division has several sections with attributes such as name of head, number of staff, number of PCs etc.
Hierachical model is a type of tree structure as shown already in Figure 4.3 (a). A set of links connect all record types in a tree structure.
The advantages of hierachical model are high speed of access to large datasets and eases of updating. However the disadvantage is that linkages are only possible vertically but not horizontally or diagonally, that means there is no relation between different trees at the same level unless they share the same parent.
The Quadtree, that is used to access a small part of a large raster image or map area, is a type of hierachical model. Quadtree first divides a total map area into 4, 16, 32, .... step by step as shown in Figure 4.4 (a).
Secondly a quadtree is built as shown in Figure 4.4 (b), that makes the access to a particular area at high speed. Numbering of 0, 1, 2 and 3 known as Morton order, makes effective coding of a block or a pixel in a raster model. For example the block of 211 in Figure 4.4 (a) can be expressed 100101 in a pair of base 2 digits, while the conventional block number (4, 3), line and row number needs more bits in a computer.