Geographical information system

Geographical information system

Introduction

Geographical information systems (GIS) are designed to store? retrieve? manipulate? analyze? and map geographical data. The central element of a GIS is the use of a location referencing system so that data about a specific location can be analyzed in its relationship to other locations. Both plane and global coordinate systems are commonly used. A system may be capable of easily transforming one referencing system (e.g. Universal Tranverse Mercator (UTM)) to some other referencing system (e.g. State Plane Coordinates). (Liebman, 1976, 102) This makes it possible to take data that has been stored in one form and combine it with data that has been entered and stored in some other form. The use of GIS has come of age as a result of several interrelated factors. (Arentze, 1996, 126)

First? there are many GIS software products that are available from commercial vendors and universities. Second? computer workstations are now capable of handling many of the computational? retrieval? and storage problems within a reasonable amount of time and at reasonable cost. Third? graphical displays and plotters are now sophisticated and fast? producing high-quality and high-resolution output. Fourth? geographic data vendors as well as governmental agencies such as the Bureau of the Census of the US Government have made large amounts of geographic data available at reasonable cost. Fifth? the use of remote sensing has expanded? especially in environmental monitoring? and this has led to the need for systems that are capable of handling large amounts of data as well as serve as a major source of land coverage information. Sixth? the emergence of the satellite based Global Positioning System (GPS) has made it easy to collect attribute data along with its location at relatively low cost and with relatively high accuracy. Each of these factors has contributed to the growth of the GIS industry. (Liebman, 1976, 102)

Background

There are two fundamentally different types of GIS software. They differ in terms of the data model? i.e.? the means for storing geographical data. Since the real world is so complex? it would take an infinitely large database to capture the real world precisely. Data therefore must be generalized or abstracted to reduce it to some manageable quantity. (Lindquist, 1994, 4)

Data are represented as a finite set of objects. The two principal data models are raster and vector. The raster model divides the study area into a regular grid of cells? each cell contains a single value. One set of cells and associated values is a layer and a database typically contains many layers (e.g.? land cover? elevation? soil type? etc.). (Carver, 1991, 321)

Fig. An example of raster and vector-based systems.

The use of one data structure instead of another is based upon a number of factors:

(1) the form in which the data are collected or purchased;

(2) the type of analysis and models for which the data may be applied;

(3) the cost of the GIS system and data entry;

(4) the type of equipment that may be needed to support the software; and

(5) the type ...