Pixel value sum per buffer is incorrect

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10-28-2013 06:50 PM
ChristopherBride
New Contributor III
I have a 100m^2 raster of population and a series of buffers: 1,2,3,4,5,10,15,20,25,30,50 km. I am tasked with finding the population per buffer, which I am doing by using the "zonal statistics as table" geoprocess. The pixel value (population) sum decreases between 2km and 3km, and even further 3km to 4km. It then jumps way up to what I would expect to be normal at 5km. I have not gone beyond that because these results are unusable. I have verified that the projections between layers are consistent. I upgraded to the latest service pack and got identical results upon reprocessing. Can anyone help me out with this? This is a seemingly simple and straight-forward process. I've also verified that the scale is all positive (not centered on 0 which would allow negative numbers).
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16 Replies
KeaganAllan
New Contributor III
Hi Christopher,

Just a quick question, how were the buffers generated?
Have you tried using the straight Zonal Statistics using the buffers as the input feature?

Thanks,
K
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ChristopherBride
New Contributor III
thanks for responding. The buffers were generated through the buffer geoprocess, one by one. Another question I will post later is "why won't 'zonal statistics as table' work on multi-buffers? i have not triedthe straight zonal stats. I'll look into it.
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KeaganAllan
New Contributor III
Hi Christopher,

I am not a "Zonal Statistician", but just from working through some of my own examples, it seems that the way the Zonal Statistics to Table geoprocessor looks at the overlaps in the input and will report on only the the overlapping areas.

In this case, your 50km Ring Buffer may be the only one it is giving a result to.
A solution - albeit a long process - would be to cut out the smaller buffers from the larger buffer resulting in a single buffer dataset containing a series of "Donut" Polygons. (This is assuming you have the buffers in a single dataset - either merged buffers or buffers created in the multi-ring buffer tool).

This dataset could then be used to run the Zonal Stats to Table and then using Excel or a similar spreadsheet application calculate the cumulative population stats per polygon.

If I have missed your question, I apologise.

Let me know how it goes, I enjoy hearing how some of the tools in the toolbox are being used.

Cheers,
K

PS - Sorry something occurred as I re-read your original post...it could be the cell size of you Raster dataset? 100m^2 may result in some errors in the smaller buffers?
Have you checked the larger buffers to make sure?
Maybe try dropping the cell size of your raster and see if that makes a difference?
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ChristopherBride
New Contributor III
(yes, I am sitting on top of this thread to find a solution!)

All of the buffers are already dissolved, so that essentially I am dealing with discreet, non-intersecting polygons. The layer I used that has intersecting polygons accurately (as far as I could tell) sums of the population. Kinda odd. The problem of course is that i cannot add the population sums together because a huge portion of them would be counted 2,3,4 times over. Also, the individual processes should have no knowledge of the other buffers, they aren't present as an input.
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KeaganAllan
New Contributor III
Hi,

I know the feeling.
I was assuming the point was coming from a single point and not a series of points.
Unfortunately I cannot get to my PC, but in the morning I will have a look and see what I can come up with.

Would it be possible to post your buffers / section of the buffers for me to look at?
Regards,

K
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ChristopherBride
New Contributor III
I really wish I could attach the shapefiles, but the data is proprietary and I had to sign a non-disclosure agreement to get this gig (its with an NGO)
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ChristopherBride
New Contributor III
Update: I systematically chose the previous buffer to create the next one. I was doing 30 buffers, and I lost track of what I was doing. Thanks for your input! It was user error.
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KeaganAllan
New Contributor III
Glad to hear you came right.

Just a thought, would it be possible to set up the whole process in model builder to avoid user error?

Good luck with the work.

Cheers
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ChristopherBride
New Contributor III
I've never done that outside the classroom. What a great idea! I don't really know that much about it, but I am learning a lot in this whole process. I can set it up for one buffer. How would I set it up to loop the process for all 10 buffers and then do it again for a second raster?
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