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A GIS has four main functional subsystems. These are:

a data input subsystem;

a data storage and retrieval subsystem;

a data manipulation and analysis subsystem; and

a data output and display subsystem.

Data Input

A data input subsystem allows the user to capture, collect, and transform spatial and thematic data into digital form. The data inputs are usually derived from a combination of hard copy maps, aerial photographs, remotely sensed images, reports, survey documents, etc.

Data Storage and Retrieval

The data storage and retrieval subsystem organizes the data, spatial and attribute, in a form which permits it to be quickly retrieved by the user for analysis, and permits rapid and accurate updates to be made to the database. This component usually involves use of a database management system (DBMS) for maintaining attribute data. Spatial data is usually encoded and maintained in a proprietary file format.

Data Manipulation and Analysis

The data manipulation and analysis subsystem allows the user to define and execute spatial and attribute procedures to generate derived information. This subsystem is commonly thought of as the heart of a GIS, and usually distinguishes it from other database information systems and computer-aided drafting (CAD) systems.

Data Output

The data output subsystem allows the user to generate graphic displays, normally maps, and tabular reports representing derived information products.

The critical function for a GIS is, by design, the analysis of spatial data.

It is important to understand that the GIS is not a new invention. In fact, geographic information processing has a rich history in a variety of disciplines. In particular, natural resource specialists and environmental scientists have been actively processing geographic data and promoting their techniques since the 1960's.

Today's generic, geographic information system, is distinguished from the geo-processing of the past by the use of computer automation to integrate geographic data processing tools in a friendly and comprehensive environment.

The advent of sophisticated computer techniques has proliferated the multi-disciplinary application of geo-processing methodologies, and provided data integration capabilities that were logistically impossible before.