Re: [Jts-topo-suite-user] Fwd: speeding up unions/intersections with bounding boxes

[Dave P. <dav...@pi...>]

Hi Martin,

Thanks for the advice I will have a play this weekend.

I am using small polygon to represent the possible movement off an object,
 I union these small polygons and calculate the areas where your allowed
to travel by deriving intersection  with a much larger polygon which
defines which areas your allowed to travel in.

I think I will have a play with the PreparedGeometry and see what happens.

Dave.

> Interesting question....
>
> Without seeing your exact data, it's hard to be specific, but here's some
> comments:
>
> - The JTS intersection method does check for bounding box intersection
> initially.  Union does as well, in order to carry out trivial union if
> possible
>
> - In general it's probably not efficient to clip a polygon to the bounding
> box of another polygon before doing the full intersection. However, if you
> are intersecting a lot of small polygons in the same general area to a
> much
> larger polygon (e.g. a study area against a coastline), it would be
> worthwhile clipping the large poly to the overall envelope of the smaller
> ones
>
> - Another thing you can do in the above situation is to use
> PreparedGeometry.intersects() as a pre-test for the possibllity of
> intersection.  Prepare the large polygon, then check intersects on the
> smaller ones before actually computing the intersection.  You can also
> check
> covers() - if that is true then the intersection is just the small polygon
> itself.
>
> (It would be nice to add an intersection() method to PreparedGeometry
> which
> performs all these checks automatically.  I'll add that to the ToDo list)
>
> If you do try any of the above, it would be great to hear back about what
> worked and how well.
>
>
> Martin
>
>
> On Thu, Mar 24, 2011 at 12:06 AM, Dave Potts <dav...@pi...>
> wrote:
>
>>
>>
>> I do a lot of processing with the unioning of polygon geometries and the
>> intersections of a coastline polygon.
>>
>> I was just wondering if I applied a bounding box to the intersection or
>> unionioning construct to clip the area that would have qny effect on the
>> speed processing?
>>
>> Is  jts already smart enough to do this type of thing by default?
>>
>>
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