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Buffer Density Analysis

Question asked by mikeldub on Aug 8, 2012
Latest reply on Aug 9, 2012 by Caleb1987

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Hello All,

I am running an analysis to prioritize locations that have the highest buffer densities, ie places where many buffers overlap. Any idea how to determine this? I don't think it's feasible to separate each buffer into its own layer.

Any help would be much appreciated,

thanks,
mike

Caleb1987 Aug 8, 2012 10:36 AM

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One thing you can do (this is more ideal for points) depending on the scale of your buffers is to convert those to points then to rasters (you will have to play around with the cell size depending on your scale) and run focal statistics using the sum.

Before converting the buffer points to raster, you need to add a field and give all the points a value of 1 and use this as the value field of the raster. Once you convert this to a raster you can run focal statistics using the sum to show areas with high densities of buffers. In order to get the effects of your buffers you will need to set the neighborhood to something that would be close to the size of your buffers to find out how many overlap. I have never tried to do this with polygon features and you will probably have to play around with cell sizes and neighborhoods to get good results. Hopefully this will be a good place to start.

Caleb

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Caleb1987 Aug 8, 2012 10:36 AM
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One thing you can do (this is more ideal for points) depending on the scale of your buffers is to convert those to points then to rasters (you will have to play around with the cell size depending on your scale) and run focal statistics using the sum.

Before converting the buffer points to raster, you need to add a field and give all the points a value of 1 and use this as the value field of the raster. Once you convert this to a raster you can run focal statistics using the sum to show areas with high densities of buffers. In order to get the effects of your buffers you will need to set the neighborhood to something that would be close to the size of your buffers to find out how many overlap. I have never tried to do this with polygon features and you will probably have to play around with cell sizes and neighborhoods to get good results. Hopefully this will be a good place to start.

Caleb
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Caleb1987 Aug 8, 2012 11:12 AM
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Here is a quick analysis I just did. I have used focal statistics to identify the number of structures within 150 ft of each other. This method should give you a pretty good idea of the spots that should be high priority.

[ATTACH=CONFIG]16781[/ATTACH]

Caleb
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mikeldub Aug 9, 2012 5:23 AM
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Thanks Caleb,

that makes perfect sense, but when I tried to convert the buffers to points, I got the following:

WARNING 000117: Warning Empty Output Generated

Should I not be using the Feature to Point tool?

-m
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Caleb1987 Aug 9, 2012 5:33 AM
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Mike,

Hmm that is strange. I would have gone with the feature to point function as well, but I don't know why you would be getting that error message. Were your buffers based off of a point feature class? If so, I would just use the original point FC to convert the points to raster. Also, what is the size of your buffers?

Caleb
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mikeldub Aug 9, 2012 5:44 AM
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No, buffers were originally based on polygon features.

Buffers are 1/4 mile.
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Caleb1987 Aug 9, 2012 5:58 AM
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Did you try using the original polygons as the input for the Feature to Point tool? I don't understand why the buffers did not work for you though.

If that does not work either for some reason I have one other idea. You could also try converting the original polygon features to raster if your polygons are fairly close in size. You would want to set a cell size slightly larger than that of the largest polygon to ensure that you only get one cell per polygon.

If neither of the above works for you, would you mind attaching a small sample of the data?

Caleb
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mikeldub Aug 9, 2012 6:31 AM
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I converted the original polygons to points and got a blank raster when I tried the feature to raster tool.

A sample of the data is attached (it's in xls format since the forum doesn't allow csv uploads).

Thanks again for your help with this.
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- publicFac_sample.xlsx16.9 KB
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Caleb1987 Aug 9, 2012 6:47 AM
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No problem. When you converted the points to raster what cell size did you use? The output raster had no cells at all? Also, instead of using the feature to raster tool it may be better to use the point to raster tool. One more thing, before converting the points to raster did you create the value field (all = 1) and use that as the value field for the raster?

And sorry I wasn't very clear about the data sample. Could you attach a sample of the shapefile with just a few of the polygons? The .xls did not have any coordinate info. I just want to take a look at the data to see if I can figure out why these things are not working for you. Thanks

Caleb
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mikeldub Aug 9, 2012 8:20 AM
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I tried the default cell size of 200 and 60 and neither produced a raster with any cells at all. I tried the same with the Point to Raster tool and got the same results. Yes, I included a field called 'weight' with values of 1.

There is a sample of the polygons and their buffers attached.

thank you
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Caleb1987 Aug 9, 2012 8:52 AM
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Mike,

I am not sure why you are having this problem. I was able to get it to work. However, I only have an arcEditor license so I do not have the feature to point capability. To get the buildings to centroids I used the "points to centroid" tool in the xTools Pro extension that I have. However, as far as I know this tool works the same as the features to point tool so I am not sure what is happening. I was able to convert it to raster without any problems.

The first one I did I used a cell size of 200 for the polygon centroids and I used a focal stats circular neighborhood of 1320 ft to get the 1/4 mile effect of the buffers. Here is what it gave me:

[ATTACH=CONFIG]16803[/ATTACH]

I did a second one as well just for display purposes using the same polygon centroid raster and used a circular neighborhood of 5000 ft.

[ATTACH=CONFIG]16804[/ATTACH]
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- density1.jpg66.2 KB
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mikeldub Aug 9, 2012 9:30 AM
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Thanks for all your help Caleb,

I don't know where my issues are coming from, but I'll continue to troubleshoot. I know that what you suggested is the logical answer, so I'll probably just start from scratch and try again.

best,
mike
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mikeldub Aug 9, 2012 12:01 PM
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My problem is definitely in the conversion to raster. I'm continually getting a result with ZERO raster cells from the points.
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Caleb1987 Aug 9, 2012 12:59 PM
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I do not know why that is happening to you. I cannot imagine how frustrated you must be. Sometimes, this usually only happens when I try to run spatial analyst tools, but I will repeatedly try to run a tool and no matter what it doesn't work. When this happens I will open up a new blank mxd and add just the input data that I need and try to run the tool again. And for some reason, sometimes it will work, even though I have done nothing different from when I tried in the original mxd (might be a bug in 10?).

So I guess if you haven't tried this already, I would try to open a blank mxd and just add your point file and try the point to raster tool again. If this does not work, I would contact Esri to figure out what is going on. I hope this works. Sorry to hear you are having such bad luck!!

Caleb
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