[postgis-users] oddity with computing areas

[Eric McKeeth]

Hello. I'm seeing behavior I don't understand when computing the area
of the intersection of two geometries. Given the columns geom_a and
geom_b, both MULTIPOLYGON geometries with the same SRID (4269 in this
case), I'm selecting st_area(st_intersection(geom_a, geom_b)). In most
cases, I'm getting results that look correct. But in a small
percentage of cases, I'm getting cases where the area is much larger
than expected (i.e. as much as 30x greater than the larger of
st_area(geom_a) and st_area(geom_b)). It seems to me that it should
always hold that the area of the smaller of two geometries should be
the upper bound for the area of their intersection. I'm only seeing
the issue in cases where the two geometries intersect, but neither
contains the other. Does anyone have any ideas as to what could cause
this?

Thank you in advance for any help,
-Eric