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I've been asked to do a favor for someone since I'm "the mapping guy" and could use some theoretical help by way of examples since my actual cartography skills are a little rusty.

I have a series of points (~200) that represent events within a city over the course of a year. These events occur all over, but it's common that some points will overlap (e.g. events occurred at the same location).

I'd like to display all of the points, but in the case of overlapping points would like to highlight the fact that the frequency of events is higher at those locations. I can think of a few ways to do this (size, color), but could use some examples or information on techniques that I could use to turn this data into an informative map.

I'm currently using ArcGIS 10 to project and display the points.

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The "duplicate" question doesn't provide many examples - only a few based on clustering. Could this be re-opened so that more people could contribute ideas? –  Nick Ochoski Nov 1 '11 at 21:15
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re-opened based on more 'contribution ideas' please. –  Mapperz Nov 1 '11 at 21:19
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@Mapperz As long as this question remains a duplicate, it needs to be merged with the duplicate. We can bump the dupe (by editing it in some immaterial way) and solicit additional answers if that's desirable. Barring that, something in this current question has to change significantly in order to keep it open. –  whuber Nov 1 '11 at 21:21
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Do the overlapping points have identical coordinates or simply overlapping symbology? –  blah238 Nov 1 '11 at 21:56
    
Identical coordinates. –  Nick Ochoski Nov 1 '11 at 22:19
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5 Answers

up vote 4 down vote accepted

What about using "collect events" from the spatial statistics toolbox; I think this is available at all license levels?

This will automatically count overlaps and symbolise with graduated markers, but you could use the count to symbolise any way you like.

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I ended up using Collect Events with Rendering. Good quick solution that can also be used to create heat maps given a few more steps as pointed out in another answer. –  Nick Ochoski Nov 7 '11 at 17:32
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Unfortunately, that other question is about web maps, which have a much more limited way to symbolize data.

With ArcMap 10, you have quite a few options. You could use graduated or proportional symbols, but that goes back to the color/size method you already mentioned. And truthfully, based on what you are describing, I think some form of chart symbology may be the most useful. You will likely need to create another field in your attribute table in order to get the results you want however.

I would probably populate this new field with integer values that represent the number of overlapping points (E.g., 3 overlapping points gets a value of 3). Then when you symbolize based on this field, your chart will give proportional weight to areas with high/low occurrence of events. A bar or stacked chart will probably display better than a pie chart.

Also, if you are concerned about multiple points trying to display charts at the same X/Y, you could just populate one of the coincidental points with a value and leave the others as zero. Then just use a definition query to not display the zeros, and that leaves all the rest that have >0 values.

By using this method, I'm assuming you aren't trying to distinguish individual events, but mostly trying to show intensity at a location?

It can be very difficult to cleanly show multiple symbols for points that occupy the same spatial location. That's why the other "duplicate" question answers mention clustering. Clustering can look really messy, though, especially if other points are nearby, but not the same X/Y.

Let me know if you need any clarification, or if I didn't answer your question well.

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Thank you for pointing out the difference between this question and its near-duplicate. –  whuber Nov 2 '11 at 15:17
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I would use an opacity level to deal with this. For example if you set the opacity (often referred to as an alpha level) to be 5; then if 5 points are completely on top of each other you would get a fully opaque dot.

An alternative approach could be to calculate the density of the points in ArcGIS (there is a function to generate a raster based on a few parameters such as raster size, interpolation type, etc). This would give you a surface which you could use, or trim to the areas where the points are.

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I've had a need for a similar tool before too, but never took the time to put something together. So you're question made me think of ways I would do it, and the basics of what I came up with is:

  1. Get a frequency of points sharing the same spatial location
  2. Join the frequency back to the original point table
  3. Then, as @Baltok suggested, use graduated symbols or colors to assist in the visualization.

It took a couple more steps than that, but I created an Arc10 model (works with all license levels) that accomplishes the task. My model leaves all of the original features and just adds a "Frequency" field to the end of each record that you can symbolize on. It does not take into account "clustering" of points within a distance, just purely overlapping.

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I'm sure this is a good tool, but unfortunately Join Field within the Model is at ArcInfo Licence level. So the model doesn't work if you don't have ArcInfo. I have tried to substitute this with another script - "JoinFieldArcView" but no joy yet –  Stev_k Nov 3 '11 at 11:22
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@Stev_k, Point taken. I looked at Join Field a bunch of times before posting and I could have swore it was AV license. Anyway, I have updated the model to use an "Add Join" tool in its place. There is one tool labeled with "(ArcView)" and another "(ArcInfo)". The ArcView version should work just fine for you now. –  RyanDalton Nov 3 '11 at 15:00
    
It works. Excellent tool, thank you. –  Stev_k Nov 3 '11 at 17:58
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A heatmap algorithm could eventually be applied to display the intensity of events. You will loose precision regarding events location but you will provide a better representation of hot areas in the city.

I'm not an ESRI guy, but I'm sure some heatmap libs are already available.

See some examples online :

If heatmap can fit your needs, have a look on this Q&A.

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