[postgis-users] Buffer query

[Bill Tenbroeck]

I am trying to generate a query that will yield the areas of US blockgroups that fall within a 5-mile radius of a point. The area should be clipped at the radius boundary, and only return that portion within the buffer. My projection is Albers, North American. It would *seem* that the sum of the resultant output would yield the area of a circle with a 5 mile radius. (In square meters, I have calculated this to be a bit over 203 million sq meters). However, my results are summing to a little over 181 million sq meters.
 
The query I am using is:
 
SELECT area(intersection(r.geom, buf_geom)) AS clippedarea, area(r.geom) as totalarea, r.fips, '27653' as CID
FROM blkgrp_alb AS r, Buffer(GeomFromText('POINT(1919020.564 673785.6216)',102008),8046.72) buf_geom
WHERE r.geom && buf_geom
AND intersects(r.geom, buf_geom)
order by fips
 
Can anyone see either the flaw in the query or my logic?
 
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