This kind of question can be answered by a classic N-polygon overlay.
AFAIK PostGIS doesn't have such a function directly, so you will have to
compute the noding of the polygon linework, followed by a polygonization
step. Then take a interior point of each resultant polygon and compute
how many source polygons it lies in. You can then union the set of
polygons for each count of interest.
Be warned - if the sample below is representative of your data, you may
well encounter TopologyExceptions, since it appears that there is a lot
of nearly coincident linework.
No doubt others can weigh in if there is a better way of doing this in
PostGIS.
On 7/30/2013 9:37 AM, Sparr wrote:
It was suggested on IRC that I ask this question to this mailing list.
I have a set of 1000 (or many more) polygons, most of which intersect
with each other. I want to produce a polygon (or set of polygons, or
an empty set) describing the area covered by N-or-more of those
polygons. For N=1, the answer is the union of all of the polygons. For
N=1000, the answer is their intersection. The naive way to calculate
N=2 through N=999 is to make a list of combinations of polygons,
intersect those, and then union the results, but this will require a
prohibitively large number of union and intersection operations in
some cases.
To illustrate the problem, here is an example with a smaller set of
data (422 polygons right now):
http://regionaldifferences.com/results.html?region=New%20England&lat=42&lon=-73&zoom=6
If you hover your mouse over the center of Massachusetts you'll see
that 98% of the polygons (415/422 currently) intersect there. Syracuse
NY is about 25%.
What I want is the outline of specific percentiles on this map. I want
a single polygon representing the 50%-or-more area, and a single
polygon representing the 90%-or-more area, etc.
Can PostGIS do this without my needing to assemble a string of union
and intersection operations for the query? I would prefer not to
rasterize the data unless it proves infeasible to do this with the
full precision vector data.
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