[postgis-users] Simplify() and direction dependency

[Markus Schaber]

Hi, Martin,

Martin Davis schrieb:

> 2) Another problem with using raw D-P (and most other straightforward
> simplification algorithms, including the one you mention) is that they
> do not ensure that there are no intersections in the simplified
> linework.  To do this requires a much more sophisticated approach, which
> performs global checks to ensure that no intersections are created.
> (Markus, have you encountered this problem at all?)

I also encountered this problem, but JTS exposed a function to create
valid polygonal gemetries out of linework, wich I used to re-convert the
geometries. Back then, AFAIR JTS did not include the D-P based
algorithms it has now, so I used my homegrewn algorithm. I did not yet
use D-P with this approach, so I did not know whether D-P also has this
problem.

Additionally, our data base has changed. Current NavTeQ data releases
contain the country borders as linework. So we may create a fully noded
multiLineString out of it, and then use a simple D-P on the lines it
contains (as we don't need the polygonal aspects).

For oceans and other water polygons, we likely will use the new
TopologyPreservingSimplifier from jts, and then manually drop small
rings and polygons.

> Ultimately, a goal for JTS is to provide a Coverage datatype.  This
> could be easily used in conjunction with the existing
> TopologyPreservingSimplifier code to allow straightforward coverage
> simplification.

I'm looking forward to it!

Thanks for your good work,
Markus
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