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I am looking to create an output that consists of an 11KM buffer around a number of discrete points, where the buffer has a value defined by a column in the data sets attribute table.

Is this possible?

If so, is it possible to aggregate the buffer values that 'overlap'?

The points represents the origin of each buffer, where the buffer has an 11Km radius. I would like to assign each circle (I have made the fill 95% transparent to simulate, visually, what I am looking for) a value equal to a column in the attribute table, and then aggregate the values wherever two or more buffers overlap/ So, for instance, the two buffers on the souther edge of the city would have an aggregate value equal to the sum of each of those buffers. The buffers in the middle of the city would have an aggregate value equal to the sum of all overlapping values. In essence, the darker the shade, the higher the value.

Does this make sense?

Red dots represent the points around which the grey buffers (at 11km each) are created.  The underlying polygons are City of Edmonton neighbourhoods.

I was able to add a buffer to the points (as described below) and then by joining the buffer values to the attribute table I was able to define a value for each buffer and visualize, as seen in the following image. Now, I am uncertain how to aggregate the values of each buffer with those that overlap it. I suspect that I need to create a raster of values, but am uncertain.

Can you suggest a method?

Individual values assigned to each buffer - seen here as 'low', 'medium', and 'high'

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3 Answers 3

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From the menu choose: Vector --> Geoprocessing tools --> Buffer(s)

enter image description here

To aggregate, check box 'Dissolve buffer results'.

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Since you already have buffers, and assigned values to each buffer, then just need to aggregate the values of overlapping buffers. I think you have two ways:

  1. Split buffers into pieces when they overlap. It was already asked in this question. Hope it will work for you. Then you get overlapping pieces, and you can use Spatial join to help you to aggregate values.

  2. Rasterize buffers. You need to apply Convert vector to raster separately for each buffer (or several buffers which don't overlap), the buffer pixels will have values, surrounding will be 0. Maybe firstly you need to create several shapefiles for each group of non-overlapping buffers. THen just sum all these rasters into one. This was also suggested in this question.

Or, if you dont want to split but to mantain circles with homogeneous value (if it has some meaning), you simply can use Spatial join just to sum values for each circle with those circles which it partially overlays (intersects).

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I don't believe so. What you would want to do is first create your 11km buffer, then do a Spatial Join to apply the attribute value in your column to the newly created buffer layer.

Based on your expanded question, I believe that you do want to use a raster approach. I can't think of a way to do what you are asking with vectors, but it should be simple raster math (addition) if you convert things over to raster.

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