usgs.govGeographic Information Systems (GIS) PosterGeographic Information SystemsPoster side 1 | What is GIS? | How does a GIS Work? | What's Special About a GIS? || Framework for Cooperation | For More Information |Poster side 2 | Applications of GIS | GIS Through History | GIS Display Techniques |Poster side 1Geographic information system (GIS) technology can be used for scientific investigations, resource management, anddevelopment planning. For example, a GIS might allow emergency planners to easily calculate emergency response times inthe event of a natural disaster, or a GIS might be used to find wetlands that need protection from pollution.What is a GIS?A GIS is a computer system capable of capturing, storing, analyzing, and displaying geographically referenced information;that is, data identified according to location. Practitioners also define a GIS as including the procedures, operatingpersonnel, and spatial data that go into the system.How does a GIS work?Relating information from different sourcesThe power of a GIS comes from the ability to relate different information in a spatial context and to reach a conclusionabout this relationship. Most of the information we have about our world contains a location reference, placing thatGeographic Information Systems (GIS) Posterhttp://erg.usgs.gov/isb/pubs/gis_poster/ (1 of 31) [8/16/2004 2:52:39 PM]information at some point on the globe. When rainfall information is collected, it is important to know where the rainfall islocated. This is done by using a location reference system, such as longitude and latitude, and perhaps elevation. Comparingthe rainfall information with other information, such as the location of marshes across the landscape, may show that certainmarshes receive little rainfall. This fact may indicate that these marshes are likely to dry up, and this inference can help usmake the most appropriate decisions about how humans should interact with the marsh. A GIS, therefore, can revealimportant new information that leads to better decisionmaking.Many computer databases that can be directly entered into a GIS are being produced by Federal, State, tribal, and localgovernments, private companies, academia, and nonprofit organizations. Different kinds of data in map form can be enteredinto a GIS (figs. 1a, 1b, 1c, 1d, 1e, 1f, and 2). A GIS can also convert existing digital information, which may not yet be inmap form, into forms it can recognize and use. For example, digital satellite images can be analyzed to produce a map ofdigital information about land use and land cover (figs. 3 and 4). Likewise, census or hydrologic tabular data can beconverted to a maplike form and serve as layers of thematic information in a GIS (figs. 5 and 6).Figure 1a. U.S. Geological Survey (USGS) digital linegraph (DLG) data of roads.Figure 1b. USGS DLG of rivers.Geographic Information Systems (GIS) Posterhttp://erg.usgs.gov/isb/pubs/gis_poster/ (2 of 31) [8/16/2004 2:52:39 PM]Figure 1c. USGS DLG of contour lines (hypsography). Figure 1d. USGS digital elevation (DEM).Figure 1e. USGS scanned, rectified topographic mapcalled a digital raster graphic (DRG).Figure 1f. USGS digital orthophoto quadrangle (DOQ).Geographic Information Systems (GIS) Posterhttp://erg.usgs.gov/isb/pubs/gis_poster/ (3 of 31) [8/16/2004 2:52:39 PM]Figure 2. USGS geologic map.Figure 3. Landsat 7 satellite image from which land coverinformation can be derived.Figure 4. Satellite image data in figure 3 have beenanalyzed to indicate classes of land uses and cover.Geographic Information Systems (GIS) Posterhttp://erg.usgs.gov/isb/pubs/gis_poster/ (4 of 31) [8/16/2004 2:52:39 PM]Figure 5. Part of a census data file containing addressinformation.Figure 6. Part of a hydrologic data report indicating thedischarge and amount of river flow recorded by aparticular streamgage that has a known location.| Top | Table of Contents |Data captureHow can a GIS use the information in a map? If the data to be used are not already in digital form, that is, in a form thecomputer can recognize, various techniques can capture the information. Maps can be digitized by hand-tracing with acomputer mouse on the screen or on a digitizing tablet to collect the coordinates of features. Electronic scanners can alsoconvert maps to digits (fig. 7). Coordinates from Global Positioning System (GPS) receivers can also be uploaded into aGIS (fig. 8).Geographic Information Systems (GIS) Posterhttp://erg.usgs.gov/isb/pubs/gis_poster/ (5 of 31) [8/16/2004 2:52:39 PM]Figure 7. Scanning paper maps to produce digital datafiles for input into a GIS.Figure 8. Collecting latitude and longitude coordinateswith a Global Positioning System (GPS) receiver.A GIS can be used to emphasize the spatial relationships among the objects being mapped. While a computer-aidedmapping system may represent a road simply as a line, a GIS may also recognize that road as the boundary between wetlandand urban development between two census statistical areas.Data capture—putting the information into the system—involves identifying the objects on the map, their absolute locationon the Earth's surface, and their spatial relationships. Software tools that automatically extract features from satellite imagesor aerial photographs are gradually replacing what has traditionally been a time-consuming capture process. Objects areidentified in a series of attribute tables—the "information" part of a GIS. Spatial relationships, such as whether featuresintersect or whether they are adjacent, are the key to all GIS-based analysis.| Top | Table of Contents |Data integrationA GIS makes it possible to link, or integrate, information that is difficult to associate through any other means. Thus, a GIScan use combinations of mapped variables to build and analyze new variables (fig. 9).Figure 9. Data integration is the linking of information in different forms through a GIS.For example, using GIS technology, it is possible to combine agricultural records with hydrography data to determine whichstreams will carry certain levels of fertilizer runoff. Agricultural records can indicate how much pesticide has been appliedto a parcel of land. By locating these parcels and intersecting them with streams, the GIS can be used to predict the amountof nutrient runoff in each stream. Then as streams converge, the total loads can be calculated downstream where the streamenters a lake.Geographic Information Systems (GIS)