How to use buffers to estimate the attribute data for a given polygon?

Question

I have two layers: point and a polygon.

The point layer has lat/long and I have created buffers of a 1/4 mile.

The polygon layer has data such as age and income.

Is there a plug-in that can interpolate the attribute data (e.g. age or income) contained within a buffer area?

I recall this being a plug-in in ArcView, but I have no clue of its contemporary (or the process, limitations) in QGIS.

This is verysimilar to this question about apportionment: gis.stackexchange.com/questions/16358/… — RyanKDalton, Nov 8, 2011 at 0:17
I agree with @Ryan Dalton, and would merge the two questions, but unfortunately that other one is so strangely framed (as its comments indicate) that doing so would only create confusion. Unfortunately, the question here is unclear, too, due to the unusual phrase "aspatial data" and the fact that it is difficult to come up with a meaningful interpretation of "interpolate the amount" of an age. Some clarification by the OP would be helpful and welcome. — whuber, Nov 8, 2011 at 14:59
Aspatial in this case means people or median income. There is a specific plugin I am recalling from ArcView 3.x that would in fact produce the outcome I am seeking, but I can't recall its name. — rhodee, Nov 11, 2011 at 19:08

Andy Harfoot · Accepted Answer · 2012-01-05 14:52:01Z

Using Quantum GIS 1.7.3 with fTools 0.6.1 and Group Stats 1.1.1 plugins.

Load both the polygon layer (referred to as polygon) and point buffer polygons (referred to as buffer)
Ensure that the polygon layer has an attribute containing the polygon area, and the buffer layer has a unique ID attribute (use field calculator)
Use the Intersect command from the fTools plugin (Vector > Geoprocessing > Intersect) to calculate the intersection of the polygon and buffer layers.
Add a column to the intersect results containing the area of the polygons.
Use the ratio of the new area value to the original polygon area to calculate the equivalent proportion of the aspatial data for each of the intersect polygons, and aggregate these appropriately (eg SUM, MEAN etc) to get a value for each buffer ID using the Group Stats plugin.
(Optional) Join the aggregated totals back to the original point or buffer layer using the Join tab under the Layer Properties (Layer > Properties)

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