Basics of Computer File and Database Structures

Introduction

A goal of GIS is to represent and store the graphic entities of mapped information along with relevant attributes in such a way to make all the data easily retrievable and manipulable. This is done by taking advantage of the ways computers handle data in a logical fashion through file and database structures. A brief overview of the ways computers can handle data is offered here.

Simple List

In this file structure, there really is no absolute ordering of the data. The data occur in the file in essentially the same way in which they were originally entered. Simple lists may start out in a logical fashion but whenever modifications are made, they rapidly get out of order because new data are appended to the end of the list.

Ordered sequential files

They can be thought of as a rolodex (rolodex = rolling index: rotating file device used to store business contact information) in which we keep everything in alpha-numeric order. As new data are added, the file is restructured (sorted) to maintain that order.

Indexed file structures

These provide pointers to more efficiently search data. The most efficient system is to develop an index that is based on a commonly searched attribute in the database, as shown in the following figure. The search is performed on the index field. Once the record is found, the corresponding complete information is accessed through the pointer.

The software used for management and manipulation of databases is called a database management system (DBMS). The principles of data storage using DBMS are dealt with in the lesson “Data Management” in the basic level.