GEOGRAPHIC INFORMATION SYSTEM
DBMS (DATA BASE MANAGEMENT SYSTEMS)
The data bases used in GIS are most commonly relational. Nevertheless, Object Oriented data bases are progressively incorporated.
1 Hierarchical database
A hierarchical database is a kind of database management system that links records together in a tree data structure such that each record type has only one owner, e.g. an order is owned by only one customer. Hierarchical structures were widely used in the first mainframe database management systems. However, due to their restrictions, they often cannot be used to relate structures that exist in the real world. Hierarchical relationships between different types of data can make it very easy to answer some questions, but very difficult to answer others. If one-to-many relationship is violated (e.g., a patient can have more than one physician) then the hierarchy becomes a network.
Field - smallest unit of data
Segment - groups of fields; nodes of the tree structure
Data base record - a collection of related segments; a particular tree structure Data base - composed of database records
Data base description - how data base records are defined; set of assembly-language macro instructions
Root - first segment
Sequence field - one field in each segment used to order the occurrences of a given type
Fig 4.7 Hierarchical Data Case
2 NETWORK MODEL
A network model database management system has a more flexible structure than the hierarchical model or relational model, but pays for it in processing time and specialization of types. Some object-oriented database systems use a general network model, but most have some hierarchical limitations.
The neural network is an important modern example of a network database - a large number of similar simple processing units, analogous to neurons in the human brain, 'learn' the differences and similarities between a number of inputs.
Fig 4.8 Network model
3 Relational data bases
In a relational data base, data is stored in tables where rows represent the objects or entities and columns the attributes or variables. A data base is usually composed of several tables and the relations between them is possible through a common identifier that is unique for each entity. Most of the relational data bases in GIS present two variables with identifiers; one ofthem is unique and correlative, it could be numeric or alphabetic, and the second one might be repeated and helps to organize the attribute table.
The advantages of using this kind of data base are:
The design is based in a methodology with heavy theoretical basis, which offers confidence in its capacity to evolve.
It is very easy to implement it, specially in comparison with other models such as hierarchical, network, and object oriented
It is very flexible. New tables can be appended easily.
Finally, many powerful DBMS using this approach contains query languages (like SQL) which makes easy to include this tool in a GIS. Thus, some commercialised GIS packages include a DBMS pre- existent.
4 OBJECT ORIENTED DATA BASES
Based on objects, it can be defined as an entity with a localisation represented by values and by a group of operations. Thus, the advantage in comparison with relational data bases is based on the inclusion, in the definition of an objet, not only its attributes but also the methods or operations that act on this object. In addition, the objects belong to classes that can have their own variables and these classes can belong to super-classes.
Fig 4.9 Object oriented Data Base
A simple, unstructured, unordered list of data records.
Easy to construct, but inefficient to access and retrieve.
For a simple flat file with n records, (n+1)/2 search operations are required to find a record.
2). Ordered Sequential Files
Records are organized as a sequential list according to alphabetic order or other criteria.
Only LOG2(n+1) searching operations are required to find a record from the file if divide-and-conquer searching method is used.
3). Indexed files
Easy to find a specific record with associated, cross-referenced attributes.
The index is used to quickly find a particular type of information in a larger file by selecting key features that can be searched for
Direct index file
Inverted index files