Your GIS (geographic information system) is very handy and a great navigation tool, but what you see on the GIS screen isn't necessarily what the actual terrain looks like. As you use your GIS, remember the following facts:

Map Characteristic What It Means
Maps are models — not miniatures Maps generalize geographic features by using symbols so that all features will fit the specified output size.
Map scale has a huge impact on GIS analysis Small-scale maps cover large areas with little detail, and large-scale maps cover small areas with lots of detail.
Maps are a flat model of a spherical earth Maps use projections to compensate for the flat versus spherical issue, and each projection has its own type and amount of distortion.
Maps have a reference grid, or coordinate system The reference grid helps you navigate the map and links the spherical earth to the map projection.
Maps have a reference starting point, or datum. Datums are based on a model of the Earth called a reference ellipsoid and enable all the various projections in a GIS work together to give an accurate picture of the Earth.