 |
|
|
|
Maps which are input into a GIS will almost invariably be compiled with different projections, datums, and scales. In order to consistently handle these data, GIS software includes functions for converting between map projections and for precise realignment of mapped features based on selected control points identified in each of the map layers. Individual maps in the GIS may have their characteristics manipulated by renaming categories (e.g. forest & agriculture -> land; lakes & streams -> water) or using mathematical operations (e.g. feet to meters). Proximity may be tested using functions which calculate distances between features or which can create buffer zones around them. The relationship between proximate features in a map can also be calculated to provide new information. For example, pixels containing elevations in a raster dataset can be analyzed with respect to their neighbors to determine the local slope and aspect of the landscape.
The true strength of GIS, however, is in its ability to perform overlay operations between map layers. In cases where map features represent discrete categories, overlay operations can determine the intersection or union of features from different map sources. Maps representing numerical values may also be combined using mathematical relationships. As an example, a GIS may be used to find a good site for a power plant by recoding map layers for soils, slope, and proximity to cooling water and markets into suitability scores or cost estimates. These suitability maps could then be combined mathematically to create a derived map indicating the relative costs and suitabilities for building a facility throughout an entire region.
Remote sensing data, and map products derived from remote sensing, are usually critical components of a GIS. In fact, practically all maps created by the U.S. Government are based on digital or photographic remote sensing. It is beyond the scope of this module to provide a full background on GIS. Instead, this module addresses issues of compatibility between remote sensing and GIS and provides a range of examples illustrating the benefits of merging these two types of spatial analysis tools. This merging creates a synergy in which the GIS improves the ability to extract information from remotely sensed data, and this in turn keeps the GIS up-to-date with actual environmental conditions.
For further educational materials, a Core Curriculum for GIS has been developed by the National Center for Geographic Information and Analysis. Many other web sites with GIS resources may be found in the Yahoo GIS Index and the Internet GIS Resources sites.
