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Lakota Studies 400/600:  Special Topics:  Introduction to Geographic Information Systems and Science

Week 4 Notes:  Representing the Earth in a GIS

Maps are Models

Maps are models of the Earth.  They are meant to simplify, abstract, or model the Earth using colors, content, and symbols.  We cannot draw a map at 1:1 scale because it would be too large to handle.  Therefore, we must reduce the map scale, say, to 1:24,000.  We also must represent the round Earth on a flat paper map or computer screen.  Therefore, we must use a map projection to accomplish this.  We must also reference real world coordinates on a paper map or in a GIS.  We do this by devising and using a variety of coordinate systems.  The content of the map is also a model, or representation of the Earth.  We must choose what to show and what not to show, based on the needs of our project and the audience who will read the map or examine our data in our GIS.  After choosing what to show, we must choose colors and symbols to represent the features in such a way as to convey meaning, but not overly clutter the map.  

Therefore, in many ways, a map is a simplified model of the complex Earth.  That is true whether it is in paper form or inside a GIS.  In a GIS, we are representing maps as numbers--real-world coordinates, topological relationships, slopes, codes on features, and so on.  This allows us to ask quantifiable questions about the Earth via a GIS.

As you are aware already from your work in this course, the Earth is represented in a GIS as a series of layers, or themes.  A layer can be one that indicates slope of a land surface, a school bus routing network, or a grid of rainfall averages for the past 100 years.  Inside layers are geographic features.  These features have a specific shape and size.  They are also referenced to real-world coordinates.  They are linked to information in tables, tables that store their attributes.  Finally, these features have spatial relationships.  This allows you, the data user, to ask questions about which features are near others, which are connected to others, which ones contain others, which ones intersect others, and so on. 

Keep all of this in mind as you examine Earth phenomena during this course.  Remind yourself that you are working with representations of the Earth.  Your answer resulting from your analysis has much to do with how you are representing the Earth and what layers you are including in your analysis.  Ask yourself:  Would additional data have a bearing on this problem?  What kind of data would make my model more realistic--or, like the real Earth?  Sometimes, more data does not make your analysis "better."  What sorts of analytical tools and functions are you performing on the data, and are they appropriate to the problem?  

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Author:  Joseph J. Kerski, Geographer, USGS, jjkerski@usgs.gov, 303-202-4315