09-07-2012, 11:34 AM
geographic information system
6 Geographic Information SystemsNEW.doc (Size: 411.5 KB / Downloads: 28)
INTRODUCTION
Computerization has opened a vast new potential in the way we communicate, analyze our surroundings, and make decisions. Data representing the real world can be stored and processed so that they can be presented later in simplified forms to suit specific needs.
Many of our decisions depend on the details of our immediate surroundings, and require information about specific places on the Earth’s surface. Such information is called geographical because it helps us to distinguish one place from another and to make decisions for one place that are appropriate for that location. Geographical information allows us to apply general principles to specific conditions of each location, allows us to track what is happening at any place, and helps us to understand how one place differs from another. Geographical information, then , is essential for effective planning and decision making.
We are used to thinking about geographical information in the form of maps, photos taken from aircraft, and images collected from satellites, so it may be difficult at first to understand how such information can be represented in digital form as strings of zeros and ones. That problem is one of the central issues of this book is as long as it is. If we can express the contents of a map or image in digital form, the power of the computer opens an enormous range of possibilities for communication, analysis, modeling, and accurate decision making. At the same time, we must constantly be aware of the fact that the digital representation of geography is not equal to the geography itself –any digital representation involves some degree of approximation.
Since the mid-1970s, specialized computer systems have been developed to process geographical information in various ways. These include:
• Techniques to input geographical information, converting the information to digital form.
• Techniques for storing such information in compact format on computer disks, compact disks (CDs), and other digital storage media.
• Methods for automated analysis of geographical data, to search for patterns, combine different kinds of data, make measurements, find optimum sites or routes, and a host of other tasks.
• Methods to predict the outcome of various scenarios, such as the effects of climate change on vegetation.
• Techniques for display of data in the form of maps, images, and other kinds of displays
• Capabilities for output of results in the form of numbers and tables.
The collective name for such systems is geographical information systems, (GISs)
Geographic Information Systems: A Generic Definition
Many people offer definitions of GIS. In the range of definitions presented below, different emphases are placed on various aspects of GIS. Some miss the true power of GIS, its ability to integrate information and to help in making decisions, but all include the essential features of spatial references and data analysis.
From Jeffrey Star and John Estes, in Geographic Information Systems: An Introduction :
"A geographic information system (GIS) is an information system that is designed to work with data referenced by spatial or geographic coordinates. In other words, a GIS is both a database system with specific capabilities for spatially-reference data, as well [as] a set of operations for working with data . . . In a sense, a GIS may be thought of as a higher-order map."
And from Understanding GIS: The ARC/INFO Method:
A GIS is "an organized collection of computer hardware, software, geographic data, and personnel designed to efficiently capture, store, update, manipulate, analyze, and display all forms of geographically referenced information."
The term geographical information system (GIS) is now used generically for any computer-based capability for the manipulation of geographical data. A GIS includes not only hardware and software, but also the special devices used to input maps and to create map products, together with the communication systems needed to link various elements.
The hardware and software functions of a GIS include:
• Acquisition and verification
• Compilation
• Storage
• Updating and changing
• Management and exchange
• Manipulation
• Retrieval and presentation
• Analysis and combination
The GIS View of the World
GIS provide powerful tools for addressing geographical and environmental issues. Consider the schematic diagram below. Imagine that the GIS allows us to arrange information about a given region or city as a set of maps with each map displaying information about one characteristic of the region. In the case below, a set of maps that will be helpful for urban transportation planning have been gathered. Each of these separate thematic maps is referred to as a layer, coverage, or level. And each layer has been carefully overlaid on the others so that every location is precisely matched to its corresponding locations on all the other maps.
The bottom layer of this diagram is the most important, for it represents the grid of a locational reference system (such as latitude and longitude) to which all the maps have been precisely registered.
GIS PROCESS
A GIS organizes and exploits digital geographical data stored in databases. As we have already seen, the data include information, attributes, and relationships between features. But a database can only approximate the real world, since the storage capacity of a database is minuscule in comparison with the complexity of the real world, and the cost of building a database is directly related to its complexity. The contents of a book of 100,000 words can be stored in digital form in roughly 1 million bytes (the common unit of computer storage is a byte, defined as 8 bits; 1 megabyte is slightly more than 1 million bytes).