[GRASS5] Q. How do you find the islands for a given area?

Eric G. Miller egm2 at jps.net
Sun Feb 25 04:09:17 EST 2001

On Sat, Feb 24, 2001 at 07:04:15PM -0800, Eric G. Miller wrote:
> > I have recently been thinking that we should maintain a cache that
> > stores the polygons in this form for drawing purposes. Cached data could
> > be read for rapid display if the timestamp on the cache is more recent
> > than the persistent data in Map_info. Pulling topological data out of
> > dig_plus to generate geometry is quite fast anyway, but would be faster
> > still if the information was cached.
> Yes, I expect this operation is going to be expensive on CPU time...

Well, good and bad news.  The good news is, with my small test vectors,
I've been able to get d.area to not fill inner island areas.  The bad
news is it really sucks CPU and is very slow.  My hardcore test for any
vector routine is running it on the geology layer I have for California.
It's got thousands of polygons and many island polys.  Well, it takes
many many minutes to draw.  Not good.  d.vect takes about 1.5 minutes.

There's probably alot I can do to speed it up, by being more efficient
about memory dupes.  The biggest killer is the search for the closest
tie point between the outer polygon and it's inner islands.  I'm doing a
real lame nested looping thing through two polygons to find the closest
points.  Maybe you'all have a better method to identify appropriate
points to tie to two polygons together at?

More later,

Eric G. Miller <egm2 at jps.net>

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