Displaying an aoristic analysis
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An aoristic analysis shows the change over time for a crime variable at a location, when the exact time of the crime is unknown. Displaying a temporal variable within a GIS is a tricky thing to do, even without the aoristic problem. The display of a temporal variable is inherently difficult given the 2-dimensional nature of crime maps. I've found a couple of possible solutions that you might like to consider.
The first solution is to remain in the 2D environment and display the temporal change in the crime in an area by using bar graphs (or similar device) to portray the changing temporal frequency of crime for an area. This is done in this image from the Eastern Suburbs of Sydney. A number of crime hotspots have been identified (numbered 1 to 9), and for each it is possible to aggregate all of the crime incidents within the hotspot. In this example, 6 months of vehicle crime has been collated and the aoristic signature has been graphed for the first two hotspots. The bar graphs attached to hotspots 1 and 2 show the incidence of vehicle crime over a 24 hour period running from midnight to midnight, hour by hour. We can see that hotspot 1 has the highest incidence of vehicle crime before and after midnight, while the aoristic signature for hotspot 2 is the almost reverse, peaking during the middle of the day.
An alternative to the 2D display is to create an animation of the same study showing the changes in crime as time passes. This can be done fairly cheaply if you have a GIS, as acceptable animation software is available for free over the net. Generate a map of crime distribution for each time period, and save the map as a graphic file. For quality of image, I suggest a TIFF (*.tif) type image. If you are charting temporal change over 24 hours, then you will generate 24 images.
Then use animation software to compile your images into either an AVI of animated GIF file. Animated GIFs work for very small files, but AVI movie files tend to be better, and much better for PowerPoint displays. Only downside is that they tend to have large file sizes. MPEG would be best, but decent AVI to MPEG software is not free (yet). Before I had a bit of money to spend on such things, I used to use Fast Movie Processor which was both good and free. It would appear to be shareware now, and renamed VideoMach. Web links are notorious for going dead, so I suggest that you visit www.google.com and search for either of these names there. Alternatively, go to www.tucows.com and search for Movie Editors.
The animation on the right shows the 24 patterns of vehicle crime for two police districts in the Eastern suburbs of Sydney, Australia. The data is drawn from about 6 month's worth of data in 1997 and is of more historic than contemporary interest. The 'clock' at the bottom right shows the hour that is currently displayed, and it advances hour by hour (if for any reason it is not advancing, press REFRESH on your Internet browser). This is an animated GIF file, constructed of 24 images. They are fairly easy to make. A number of image processing programs available from www.tucows.com can make them.
If you have a fast Internet connection and would like to view a larger (700Kb) version of the animated gif, click here.
Finally, a slow winded approach, but one that might suffice if you are short of time, is to collect all of your images in PowerPoint. Then set the slide transitions to automatically advance every second or so. This is not a great solution, as PowerPoint seems to struggle with a decent refresh rate for complicated graphics, and tends towards dead slow. OK if you are in a rush I suppose, but it never looks too hot to me. Probably the least technical solution however.
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