I'm not sure I fully understand the problem 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.
Then for any percentile run a query to select the fragments represented
by that and st_union or st_collect them into your results.
-Steve
On 7/30/2013 12:37 PM, 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.
_______________________________________________
postgis-users mailing list
[email protected]
http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
_______________________________________________
postgis-users mailing list
[email protected]
http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
