2. Concepts, history and comparison with other tools

Basic concepts of GIS

The term GIS is currently applied to computerized information storage, processing and retrieval systems that have hardware and software specifically designed to cope with geographically referenced spatial data and corresponding attribute information. The spatial data is commonly in the form of "layers" (see figure below), which may depict topography, water availability, soil types, forests and grasslands, climate, geology, population, landownership, administrative boundaries and infrastructure (highways, railroads, electricity or communications systems).

Its capability of combining different map layers - in an operation known as "overlaying" - is one of the most important GIS functions. For a description of the overlay function, follow this link

Short history of GIS

During the 1960s and 1970s, new trends arose in the way maps were being used to assess resources and to plan land use. Realizing that different aspects of the earth's surface did not function independently of each other, people recognized the need to evaluate them in an integrated, multidisciplinary way. One way of doing this was to simply overlay transparent copies of resource maps on light tables and look for places where the various attributes on the maps coincided. 

This technique was later adapted to emerging computer technology. In what was known as a grid-cell or raster system, simple maps were made by printing statistical values on a grid of plain paper, overlaying the grid values and using the overprinting of lineprinter characters to produce grey scales representing the statistical values. The results of these early methods, however, were not refined enough to find acceptance among cartographers.

By the late 1970s, however, the technology of computer map-making had advanced rapidly, with literally hundreds of computer systems and programs being developed for various cartographic applications. At the same time, advances were being made in a number of related fields, including soil science, surveying, photogrammetry and remote sensing. The rapid pace of this development initially resulted in much duplication of effort in the various related disciplines. But as systems multiplied and experience was gained, the potential for linking different kinds of spatial data processing together into truly general-purpose geographic information systems emerged.

By the early 1980s, as computer technology became more sophisticated, less expensive and more widely adopted, GIS came into its own. Today, GIS systems are rapidly being established by public agencies, research laboratories, academic institutions, private industry and public utilities.

Comparison of GIS and with other computer systems

In essence, GIS is a data base management system (DBMS) specifically designed for simultaneous processing of spatial and related attribute data. A DBMS generally provides a language for analyzing data which allows users to describe to the system what they want to know with little or no attention to the mechanics or methods used by the system. A DBMS must also contain procedures for checking consistency of the data and maintaining their integrity. 

In addition to DBMS, GIS also has many capabilities similar to automated map making, computer-assisted cartography and computer graphics systems. However, as well as having a powerful capability for processing graphics, GIS must also be able to process non-graphic attributes, such as statistical data, in conjunction with the spatial data to which they are related. For example, if the user modifies the spatial data the GIS will make the necessary modifications in the related statistical database automatically. This link between the two types of data must be present if a system is to be considered a true GIS.

Although GIS differs from other tools, such as tabular data base management systems, computer graphics, and automated map making, each of these other systems is, in fact, a component of GIS. What GIS does is make it possible to integrate all of them in one operation.

One of the primary sources of geographic data used in GIS is information about the earth that is obtained through remote sensing. Remote sensing data are usually acquired either as digital satellite imagery or aerial photographs. After these images are geometrically corrected, enhanced, analysed and interpreted, the results can be fed into the GIS and integrated with other geographic data bases.

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