[Qgis-user] aerial overlay? pop around an airport

[rhoskins at u.washington.edu]

Vector > geoprocessing tools > intersect 1.7.3

I built a buffer around an airport of 10 miles. The buffer intersects 4 counties within the 10 mile radius buffer.

  The resulting layer is a join of the airport attributes with each of the county attributes resulting in 4 records. The geographic result is the a circle of radius 10 miles identical to the overlaid buffer.

How would I get an estimate of say, the county populations that are within the buffer, that is, an aerial overlay (assumes uniform distribution of population with the county)?

If I knew the fraction of the area of each county within the circular buffer I could calculate it. 
Also it would be useful to be able to have a sum (estimate) of the total population within the buffer.

Ideas?

Thanks -
Richard
Anchorage, AK