Hi Erik,
Erik Hofman wrote:
> Wouldn't it be beneficial to split up the datasets into rectangular
> chunks like the scenery parser does? In my mind it would be possible to
> favor hand crafted work over automatically created scenery quite easily
> that way. It might even speed up processing?
Generally I agree that splitting the dataset up into smaller tiles
would help. Actually I've already done that to a certain extent - but
the result still wasn't much different.
One obvious benefit of processing small tiles is in that they're
getting processed much quicker and thus you can estimate the ETA pretty
well simply by counting finished and unfinished tiles. Anyhow the
challenge is to find the "right" (TM) balance between processing speed
of the operation I'd like to pursue and the additional cost of
re-assembling the results.
In detail the issue is a bit more complex (the friendly folks @ GRASS
GIS told me why). The operation which takes so much time is detecting
intersections beween line segments when building a vector topology,
those lines which connect the vertices of land cover polygons.
GRASS searches all line segment in the vincinity of the one for which
the intersection test is being run and then tests for intersections of
the one segment in question with any of those in its vincinity. The
longer our segment in question is, the more intersection tests are
being run against this particular segment - which is quite obvious.
Now, to reduce unnecessary overhead, there's a function to split long
segments into smaller pieces, thus reducing the number of neighbouring
line segments to this very segment and, as a consequence, the number of
intersection tests for each segment.
Unfortunately this didn't help in the given case and the next step to
be done is to find out why.
Cheers,
Martin.
--
Unix _IS_ user friendly - it's just selective about who its friends are !
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