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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