Encyclopedia of Environmental Science and Engineering, Volume I and II

425

G

GEOGRAPHIC INFORMATION SYSTEMS

The term “Geographic Information System” (GIS) varies as

a matter of perspective, and ranges in scope from specific

computer software packages to software, hardware and data,

to software, hardware, data, and support personnel. The most

exhaustive definition is given by Dueker and Kjern as:

Geographic Information System—A system of hardware,

software, data, people, organizations, and institutional

arrangements for collecting, storing, analyzing, and dissemi-

nating information about areas of the earth.

While this is an all-inclusive definition, the software

packages at the heart of a GIS have their roots in the work

of two researchers at Ohio State University, Marble and

Tomlinson, in the mid to late 1960s. These men coined the

phrase “Geographic Information System,” and defined a GIS

as having the following four components:

1. Data Input and the ability to process data.


2. Data Storage and Retrieval with the ability to
   edit.


3. Data Manipulation and Analysis.


4. Data Reporting Systems for the display of tabular
   and graphic information.

Data input, storage/retrieval, and reporting were (and still

are) common to two other computer software packages—

Computer Assisted Mapping (CAM) and Database

Management Systems (DBMS) which separately managed

graphical and tabular information. The development of GIS

combined these packages to provide what many refer to as

“intelligent maps,” which are maps with extended informa-

tion. Extended information can include information from

areas such as census, tax assessment, natural resources

availability/quality, which may be linked to a map, but

are managed as part of a separate database. However, the

true distinction of a GIS is Marble and Tomlinson’s third

component—the ability to analyze spatial information. The

analysis capability enables a GIS to automatically evaluate

information from several sources as a function of their spa-

tial context. CAM systems may provide some of the same

information as a GIS through a series of separate maps, but

they require manual interpretation.

There are six basic analytical questions which many GIS

software packages are able to address, partially or in full.

These are:

1. Location—“What is it?”—What types of features
   exist at a certain place, such as “What is the popu-
   lation of a given census tract?”


2. Condition—“Where is it?”—Finding a site with
   certain characteristics, such as “Which agricul-
   tural fields are within 100 meters of a stream?”


3. Trend—“What has changed?”—Evaluation of
   spatial data as a function of time.


4. Routing—“Which is the best way?”—A variety of
   problems to determine paths through a network,
   such as finding the shortest path or optimum flow
   rates.


5. Pattern—“What is the pattern?”—A function
   which allows environmental and social planners to
   account for spatial distribution. Examples include
   the spread of diseases, population distribution
   versus urban development, or targeting specific
   consumer trends.


6. Modeling—“What if?”—Allow model develop-
   ment and evaluation, including “Monte Carlo”
   evaluation, where a variety of factors influence
   a situation, and their relationship is determined
   by varying one factor while holding the others
   constant.

A GIS is able to perform the above operations because

they maintain the topology, or spatial associations, of the

elements of their database. For example, an individual look-

ing for 617 East Central Avenue on a map is able to see that

this address is between 6th and 7th streets, on the south side

of the street. Furthermore, if the individual wants to drive

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