TABULAR DATABASES



A. INTRODUCTION

All GIS databases include links between graphic (map) features and tabular data that provides descriptive information for the graphic objects. If it were not for this link, a GIS would not exist but would be similar to computer aided design or computer mapping systems. The tabular data provides the "Information" in Geographic Information Systems.

These days most GIS's provide links to a variety of databases allowing users a choice between a host of commonly used systems. Therefore the GIS can be a combination of two distinct systems generated individually, but used for a common purpose.



DATABASE DEFINITION

A large collection of non-redundant data in a computer organized so that it can be expanded, updated, retrieved and shared by various uses and by various individuals.



ADVANTAGES OF DATABASES

Advantages of a database include the sharing of information to reduce redundancy. This reduction in redundancy improves the quality and integrity of the database allowing for easy maintenance.

The database can be stored in one location and consist of specific information. The display of the data to a particular user can take many forms depending on the needs of the user. Therefore, the internal representation of the data may be quite different from the external representation to the user.

Databases can included information in various forms:

INTEGERS

REAL (DECIMAL)

CHARACTER

DATES

IMAGES AND SOUND



DATABASE OPERATIONS

Databases allow users to sort, delete, edit, select, mathematically manipulate, and update information through the use of a query language. This language allows users in "ask questions" of the database in a standardized way.

Databases can be accessed within itself, but should also allow useres to access information through application software. In the case of a GIS, users use the GIS application software to access information linked to graphical features from the database.



TYPES OF DATABASES

TABULAR ("FLAT FILE")
HIERARCHICAL
NETWORK
RELATIONAL

TABULAR DATABASES

These databases are the simplest and consist of single files stored in a tabular form. These files have no relationships that can be built with other files. Simple spreadsheet programs that allow users to input, query, and manipulate information within a single disk file is an example of a tabular database.

HIERARCHICAL DATABASES:

These databases have a tree like structure with every node of the tree representing a different tabular file. Each file is related to one another through the link to the file above or below. Lateral links between files are not allowed.

Geographic data which is organized hierarchically fit into this model. A good Example of spatial data that can fit into this model is the U.S. Bureau of Census data in which information for one census tract has no lateral relation to another census tract, but are related to the higher levels in the hierarchy to city, county, and state.

In the hierarchical model an owner record (file) can have many members, but the member records can only be owned by one owner record.

NETWORK MODEL

In a network model, a member record can have many owners and the owner records can have many members. However, links between records of the same class cannot be made (members cannot own members).

RELATIONAL MODEL

This has become the most popular form of a database model since it has the most flexible approach to linkages between individual databases. There is no hard structure to a relational model and the form of the model is determined by the needs and concepts of the user.

In a relational database each record (file) has a set of attributes and a set of rows (entries) that form the table. Separate tables are linked, or related, to one another through a common attribute. Any attribute in the table can be used as the key link between that table and another table. As the diagram implies, links can be formed in any direction and between any node (table).

This database model allows maximum flexibility since ownership is not defined as it is in the hierarchical and network models.



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Author: R. Douglas Ramsey Doug@nr.usu.edu