[geos-devel] Questions on noding through the C interface
cooperch at cf.ac.uk
Tue Jan 22 10:03:36 PST 2013
I am looking to node a set of linestrings through the GEOS C interface.
Apologies in advance for the number of questions in this email, but I
hope together they give a good idea of my requirements.
Two caveats: (1) I need to be able to reject individual nodes at
predetermined locations, i.e., occasionally at my discretion allow
linestrings to cross at points other than their endpoints; (2) I need to
be able to work to a prespecified tolerance.
I notice that computing the union or intersection of a multilinestring
geometry with itself, automatically splits all linestrings where they
cross or touch. This wasn't what I was expecting - as presumably
returning the input unchanged would constitute a valid intersection with
itself - so can I rely on such behaviour?
In any case that doesn't help me with (1) above, i.e. I need to not
split at certain points. So instead I can intersect each linestring
individually with the entire multilinestring set, and if I get a
multilinestring back I know it must have intersected, and can decide
what to do with that intersection i.e. whether or not to keep it. (As an
aside - can I guarantee in the multilinestring returned that points will
appear in the same order as in the input linestring?)
However, does the second approach incorporate significantly greater
computation time? Self-intersecting the entire set might be done in O(n
log n) (is it?) but looping through individual segments is presumably
O(n^2) at best. How can I make it faster?
Finally how do I incorporate the tolerance parameter into the above
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