[GIS] Area-weighted calculation on an intersection

areacensuspostgis

I'm trying to learn postgis sql by performing some familiar operations from ArcGIS. Here, I want to perform an area-weighted calculation of certain populations on the intersection of a set of parcels with census blocks.

I have parcels parcels, parcel attributes parcelinfo, census blocks census, and census attributes tgr39035sf1blk.

This calculation would entail

  1. Calculating the area of each census block
  2. Intersecting census blocks and parcels
  3. Calculating the area of each polygon in the intersection of census blocks and parcels
  4. Calculating the area weighted value of each attribute (attribute value * (area of polygon / area of census block)

In order to test ST_Intersection, I tried the following, but phppgadmin craps out (no error message, a proxy error):

SELECT ST_Intersection(p.the_geom, c.the_geom) as inter from parcels p, census c;

I could sum the attributes from each block that intersects with a parcel, but that is, obviously, a seriously flawed approach. E.g., 

SELECT   sum(DISTINCT t.pop2000), sum(DISTINCT t.males)
FROM     parcels p, census c, tgr39035sf1blk t
WHERE ST_Intersects(p.the_geom, c.the_geom) AND c.stfid = t.stfid;

I'm sorry if I'm missing any essential, additional information. I'm a novice at this.

Best Answer

It turns out that I needed to add a WHERE ST_Intersects to my ST_Intersection Query, as follows:

SELECT sum(((st_area (st_intersection (p.the_geom,c.the_geom))/st_area(c.the_geom))*ci.pop2000)) AS Parcels_pop
FROM parcel_proj p, census_proj c, tgr39035sf1blk ci
WHERE ST_Intersects(p.the_geom,c.the_geom) and ci.stfid=c.stfid;

This may not matter for others who have more forgiving interfaces, but I was getting a POST / Proxy Error every time I tried st_intersection without testing if st_intersects, presumably because st_intersection needs to be constrained in order to function efficiently.

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