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Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition by Michael D. Kennedy

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Generating Features by Overlaying

Pick any point of Earth’s land area. A large number of attributes might describe that point. It may be within the area owned by a person. The point will be in some country or other. It might be part of a floodplain. It will have associated with it a certain soil type. The representation of the point in a GIS might indicate that it is part of a roadway or stream. Or its GIS representation might show it to be an oil well.

In doing GIS analysis, we are frequently interested in combinations of attributes that relate to points, lines, or areas. For example, we might be interested in those areas that have a certain land use zoning and that are also for sale. Suppose that we have one GIS layer whose polygons show zoning and another, different layer that shows properties for sale. By a process called overlaying, we can create a third layer from which we can identify those properties with the zoning we want that are also for sale.

The term “overlay” comes from the physical process of laying one map of transparent material on top of another—say, on a light table—and examining the effect of the combination. To continue the preceding example, suppose that you wanted to buy some property to start a business. The zoning had to be “B1” and, of course, the property had to be for sale. You might take a clear Mylar map of the area and use a marker to blacken all of the zones that were not B1. On a second such map, you might blacken all of the properties that were ...

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