PostGIS Cookbook - Second Edition

How to do it...

As the problem of a proportional sum is a generic problem, it could apply to many problems. We will write the underlying proportioning as a function. A function takes inputs and returns a value. In our case, we want our proportioning function to take two geometries, that is, the geometry of our buffered trail and block groups as well as the value we want proportioned, and we want it to return the proportioned value:

CREATE OR REPLACE FUNCTION chp02.proportional_sum(geometry,   geometry, numeric) 
  RETURNS numeric AS 
$BODY$ 
-- SQL here 
$BODY$ 
  LANGUAGE sql VOLATILE; 

Now, for the purpose of our calculation, for any given intersection of buffered area and block group, we want to find the proportion that the intersection is over the ...