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What should I know to introduce my learners to the construction and capabilities of GIS? 

Behind the scenes, a GIS is a database of geographically referenced information and shapes and an analysis and mapping tool.  The public and your learners will see GIS as a map that depicts what you tell it to depict.  Possibilities include political boundaries, property boundaries, zoning, community demographics, land use, natural resources, soil types, transportation entities, and relief or topography. 

Base maps often consist of aerial photography or relief maps.  Layers or overlays, when selected, appear semi-transparently on top of the base map.  As layers are selected and de-selected (turned on and turned off), the features of the map reflect the wishes of the user.  If the user is interested in depicting the aquatic features of a landscape like wetlands and waterways while also depicting current land use, the wetlands, lakes, hydrography, and land use layers are turned on and other unneeded layers are turned off.  The ability to overlay multiple layers makes the GIS a very powerful planning tool.  If a project needs to avoid certain features within the landscape and make use of others, the manipulation of the layers can identify locations that meet as many criteria as possible. 

GIS maps are typically viewed on the computer screen to take advantage of the more or less instantaneous map view changes that occur when layers are turned on or off.  Maps can be printed for presentation or for later review.  Unless the GIS is resident on the computer, the computer will need a fast Internet connection.  Maps are best viewed on large computer monitors.  It will not be possible for you to change the data in the databases of the GIS without having the database files and software available to you.