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<br /> <br />Polygons can represent features such as building footprints or political <br />boundaries. Polygons are also often referred to as areas. <br /> <br />son <br /> <br />A single feature can be represented by more than one type of data. For example, on a map of the United <br />States, capital cities could be represented by points, In a map of a single county, however, the capitol city <br />might be represented by a polygon, The above maps show what typical point, line and polygon data look <br />like. These maps would probably be much more useful if you could view all of these data types together, <br />This is the concept of data layers, which will be covered in the next part. <br /> <br />Part 4: Thematic mapping and map layers <br /> <br />Thematic mapping refers to the practice of assembling data to create a map that illustrates patterns in the <br />data. Thematic maps use distinctive colors, symbols and shading styles to portray data such as population, <br />rainfall, income level or flood zones. The different styles highlight the similarities, differences, and trends <br />among map features. <br /> <br />Thematic maps are usually organized into layers, each representing a different theme, Each layer usually <br />contains only one geographic feature or phenomenon, For example, streams, roads, well locations, and <br />political boundaries would each be stored as separate spatial map layers. This makes it easier to manage and <br />manipulate the data. The features are shaded or represented by different symbols according to the range of <br />characteristics of the features contained in the layer. Layers can be drawn directly over each other because <br />each layer uses a common geographic coordinate system. Even though each layer is contained in a separate <br />data file, you can analyze data using infonnation from multiple layers. You will use multiple layers for <br />analysis in several of the following exercises. <br /> <br />The figure below is a thematic map of the Q3 flood data for a portion of Durham County, North Carolina. In <br />the thematic map, all the areas that are Special Flood Hazard Areas (SFHAs) are shaded and the rest are <br />white. In addition to the Q3 flood data, this map also has the following layers: Durham County streets (thin <br />lines), Durham County schools (stars), and Durham County bridges (dots). <br /> <br />40f9 <br /> <br />3/17/98 10:05 AM <br />