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GIS: General Information

GIS stands for Geographic Information System, a means of storing, displaying, and analyzing data that can be referenced geographically. Maps are a common GIS product. A GIS consists of data, computer hardware and software, and of course, operating personnel.

A GIS takes data stored in traditional tabular format databases (i.e. rows and columns), and gives it geographic references so that it can be displayed graphically. A GIS can also organize data by subject matter or theme, via data sub-sets called layers. A GIS can combine many layers of information.
One of the most common reasons to use a GIS is its ability to create graphical images -- maps -- from database data. It is often easier to identify patterns in such images than in tables and rows.

Because a GIS can link data sets together, it can make data sharing and management easier. GIS can also assist with decision-making, because it helps users to analyze, query, and map the data. "Answers" can be displayed visually, allowing the user to compare different data more easily.
GIS and GPS are not the same thing, but they are related. GPS stands for Global Positioning System, a worldwide radio-navigation system of 24 satellites and their ground stations. GPS uses these satellites as reference points to calculate positions accurate to meters. These geographic position data are converted to a file that can be used by a GIS, which can map it to show the location of the actual coordinates.
To draw the Earth (or a portion of it) onto a map, map makers use different map projections that apply (or "project") the curved Earth surface onto a flat map. Although no projection will perfectly depict all areas of the Earth, different map projections make specific areas of the Earth look as accurate and realistic as possible. TOXMAP uses a map projection (called "North American Albers Equal Area Conic") which makes the continental United States look most realistic. Areas farther away from the US will appear increasingly distorted.

You can learn more about map projections from the USGS at

As with statistics, graphs, and other visual representations of data, it's important to examine the content of a map closely. Subtle details, generalizations, and personal perspectives all impact the viewer's interpretation of what the map portrays.

In his book, How to Lie With Maps [i], geographer Mark Monmonier provides users with guidelines to help them read maps critically. Some of these tips, along with others from TOXMAP, are listed below.

  • Remember that not all maps are made by professional cartographers. Today's image and cartography software have made it possible for non-cartographers to create and design maps that may hide or obscure features that many users would expect to find.

  • Keep in mind that when portraying a three-dimensional world via a map, the geographic region that the map represents may in some ways be changed or distorted. For example, map symbols are often "thicker" or larger than the elements they depict.

  • Don't forget that not all maps are designed to inform the user about a location or geographic region. While some maps are symbols for geographic knowledge, others are created just for "looks," with less or little attention to accuracy.

  • Pay close attention to the map legend and to the symbols used on the map. For maps that use color-coding to represent ranges of data values, note how the ranges are determined and how data are categorized within.

  • Learn as much as possible about the data depicted on the map. When was it created? Was it summarized or simplified in some way before being used?

  • Know that everything on a map can be considered "data" and deserves whatever attention is relevant to your work. For example, a map might use accurate data to represent large rivers and bodies of water, but have incomplete or out-of-date representations of streams, ponds, tributaries, and other small water features. While this map might be useful for the casual user, someone concerned with local flooding or wildlife would likely need better water resource data.

  • Think as much about what isn't shown on the map as what is. Are there geographic features such as mountain ranges that, if shown, would improve your understanding of the map?

  • There are many things to keep in mind when reading and interpreting a map. Don't be intimidated by the map and the information it conveys--maps can help you discover things that no other medium can. However, be sure to keep the guidelines above in mind.

    [i] Monmonier, Mark. How to Lie with Maps. 2nd edition. Chicago: University of Chicago Press, 1996.