HOW A GIS WORKS
A GIS stores
information about the world as a collection of thematic layers that can be
linked together by geography. This simple but extremely powerful and versatile
concept has proven invaluable for solving many real-world problems from
modeling global atmospheric circulation, to predicting rural land use, and
monitoring changes in rainforest ecosystems.
GEOGRAPHIC
REFERENCES
Geographic information contains either an explicit
geographic reference such as a latitude and longitude or national grid
coordinate, or an implicit reference such as an address, postal code, census
tract name, forest stand identifier, or road name. An automated process called
geocoding is used to create explicit geographic references (multiple locations)
from implicit references (descriptions such as addresses). These geographic
references can then be used to locate features, such as a business or forest
stand, and events, such as an earthquake, on the Earth's surface for analysis.
GIS TASKS
General purpose GIS’s
perform seven tasks.
• Input of data
• Map making
• Manipulation of data
• File management
• Query and analysis
• Visualization of results
Input of Data
Before geographic
data can be used in a GIS, the data must be converted into a suitable digital
format. The process of converting data from paper maps or aerial photographs
into computer files is called digitizing. Modern GIS technology can automate
this process fully for large projects using scanning technology; smaller jobs
may require some manual digitizing which requires the use of a digitizing
table.
Today many types of geographic data already exist in
GIS-compatible formats. These data can be loaded directly into a GIS.
Map Making
Maps have a special
place in GIS. The process of making maps with GIS is much more flexible than
are traditional manual or automated cartography approaches. It begins with
database creation. Existing paper maps can be digitized and computer-compatible
information can be translated into the GIS. The GIS-based cartographic database
can be both continuous and scale free. Map products can then be created
centered on any location, at any scale, and showing selected information
symbolized effectively to highlight specific characteristics.
The characteristics of atlases and map series can be encoded
in computer programs and compared with the database at final production time.
Digital products for use in other GIS’s can also be derived by simply copying
data from the database. In a large organization, topographic databases can be
used as reference frameworks by other departments.
Manipulation of
Data
For small GIS projects it may be sufficient to store
geographic information as simple files. There comes a point, however, when data
volumes become large and the number of data users becomes more than a few, that
it is best to use a database management system (DBMS) to help store, organize,
and manage data. A DBMS is nothing more than computer software for managing a
database--an integrated collection of data.
There are many different designs of DBMS’s, but in GIS the
relational design has been the most useful. In the relational design, data are
stored conceptually as a collection of tables. Common fields in different
tables are used to link them together. This simple design has been widely used,
primarily because of its flexibility and very wide deployment in applications
both within and without GIS.
Query and Analysis Once you have a functioning GIS
containing your geographic information, you can begin to ask simple questions
such as
• How far is it between two places?
• How is this particular parcel of land being used?
• What is the
dominant soil type for oak forest?
• Where are all the sites suitable for relocating an
endangered species?
• Where are all of the sites possessing certain
characteristics?
• If I build a new highway here, how will animals in the
area be affected?
GIS provides both simple point-and-click query capabilities
and sophisticated analysis tools to provide timely information to managers and
analysts alike. GIS technology really comes into its own when used to analyze
geographic data to look for patterns and trends, and to undertake "what
if" scenarios.
Modern GIS’s have many powerful analytical tools, but two
are especially important. Proximity Analysis is used to examine spatial
relationships by determining the proximity relationship between features.
Overlay Analysis integrates different data layers to look
for patterns and relationships. At its simplest, this could be a visual
operation, but analytical operations require one or more data layers to be
joined physically. For example, to analyze the impact of urbanization on
ecological characteristics of an area, an overlay could integrate data on
soils, hydrology, slope, vegetation, and land use. Queries could be used to
identify sources of pollution, to delineate potentially sensitive areas, or to
plan for increased population growth in the area.
Visualization
For many types of geographic operations, the end result is
best visualized as a map or graph. Maps are very efficient at storing and
communicating geographic information. While cartographers have created maps for
millennia, GIS provides new and exciting tools to extend the art and science of
cartography. Map displays can be integrated with reports, three-dimensional
views, photographic images, and with multimedia.
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