[postgis-users] Using PostGIS for partial intersection/union of many polygons

[Sparr]

On Tue, Jul 30, 2013 at 12:46 PM, Stephen Woodbridge <
woodbri at swoodbridge.com> wrote:

> I'm not sure I fully understand the problem


Check out http://regionaldifferences.**com/results.html?region=New%**
20England&lat=42&lon=-73&zoom=**6<http://regionaldifferences.com/results.html?region=New%20England&lat=42&lon=-73&zoom=6>
which
is a rendering of an example data set. Move your mouse cursor around and
the box in the bottom right will tell you how many of the polygons contain
your mouse cursor. Now, attempt to move your mouse such that it exactly
follows the 50%-of-polygons "contour line" of that map. That path or
contour line is what I want, although for other percentages than 50.

but can't you do all n=1000 intersection so you get all the smallest pieces
> into a table maybe adding attributes about which polygons these contributed
> to the fragments.
>

First, doing every possible two-polygon intersection with 1000 polygons
would produce 499500 result polygons, which is way too many to store, even
temporarily. Second, you would need to do every three-polygon intersection
as well, and four-polygon, etc. And then you'd need to take unions of those
results. We are talking about trillions (and so many more) of union and
intersection operations.
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