Representing fire frequency by overlapping polygons
I'm currently trying to represent fire frequency within a certain area and running into problems on how to go about this. I have a shapefile containing fire history from beginning of record (1900) to 2008. Many of these polygons overlap and what I'm trying to visually and quantitatively represent is how many times have certain areas burned. Attributes include year (e.g. 1955), fire name, etc.
Any help would be appreciated.
Mapping Center Answer:
In ArcGIS 10, you will have the option of showing these fairly easily through animation. If you really need them to be statically displayed on paper then you have a few options but probably the best is a technique called "small multiples" in which a series of displays with the same graphic design structure depict changes in variables from multiple to multiple (i.e., map to map). The consistency of design assures that attention is directed toward changes in the data.
In your case, the maps would be the extent of the area in which file has occurred and each multiple would be a unique time step. I have attached an example of this technique as it was used to show voting patterns in Oregon over time.
Other approaches include overlapping polygons but with fire, previous time periods could be obscured by later time periods. You could also substitute polygon centroids for the polygons. In this case you would lose the visual impression of the fire extent. Another option is to show all polygons using the polygon outline and labeling the extents by time period. This works if you do not have many polygons and they are not too small.
There are likely other solutions as well but without knowing more about your data, it is hard to guide you to a best choice. You could always take the route that I take and look through National Geographic magazines, atlases and other sources to see what solutions other cartographers have come up with in the past.
And I encourage you to try this out with animation when you get ArcGIS 10!
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