Hello all,
Well, I'm learning fast but I have come across a problem that perplexes
me. I have three polygon tables. "new_vegetation" represents vegetation
on a site. "grading_phases" represents the grading area for a project,
and this is entirely within the limits of "new_vegetation". The third
layer is "basin_grading", which represents five proposed basins (as well
as polygons for 100' and 200' buffers around those basins), some of
which are within "grading_phases" and some of which are outside of
"grading_phases". Some are even split between being and out of
grading_phases.
What I am trying to get is the acreage of vegetation communities
(new_vegetation) within the each polygon type of basin_grading, but only
those portions that are within grading_phases. The query below is my
attempt at this, but it returns the acreage of new_vegetation within
each entire basin, not just the portion that is also within
grading_phases. I thought the ST_Intersects part would take care of
that, but apparently not.
SELECT
f.use,
n.cat_2008,
Sum(area2d(ST_Intersection(n.the_geom, f.the_geom)))*0.000247105381
AS inside_project
FROM
basin_grading f,
grading_phases c,
new_vegetation n
WHERE
ST_Intersects(c.the_geom, f.the_geom)
GROUP BY f.use, n.cat_2008
ORDER BY f.use, n.cat_2008
thanks in advance,
Marcus
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