Hi Martin,
(Replying from my tablet so its really hard to not top-post, sorry.)
You are probably correct that sliver elimination could be causing errors
with the "point in a polygon" computation -- but this seems to be an order
problem and I'm pretty sure this wan't always the case -- but that doesn't
matter if it's a problem in the current code.
Agreed that topological consistency problems would go away if you do all
the polygon clipping with your external grass tools and none of it within
terragear -- but you can't mix them anywhere or you'll be back to having
problems again.
I'm not sure why not having a landmass is a neat side effect? That was
simply a catchall for covering areas not covered by other data -- I could
only see that causing problems if the input is not topologically consistent.
Regards,
Curt.
On Mar 13, 2012 10:23 AM, "Martin Spott" <martin.sp...@mgras.net> wrote:
> Hi Curt,
>
> Curtis Olson wrote:
>
> > So really, much of the terragear code itself is there to create a
> > topologically clean data set from a plethora of inputs from a wide
> variety
> > of sources.
> >
> > The problem is that the terragear mechanism uses the GPC polygon clipping
> > library -- and real world GIS data imposes every nasty degenerate case
> you
> > can imagine -- and does it at the limits of floating point resolution.
> In
> > some ways you can think of this as inconsistencies that are too small for
> > the GPC library to detect so that leads to non-topologically consistent
> > results.
>
> Yup, we've discussed this a lot in the past. But the GPC lib is not
> the sole cause of trouble. There are more items in the processing
> chain which, while trying to sanitize the "slivers" for example, are
> adding even more topological inconsistencies to the picture (centroids
> ending up sitting in the neighbour polygon).
>
> That's why I chose GRASS GIS as the preferred tool, simply because it's
> known to be topologically robust. I suspect GRASS probably wasn't an
> option back in these days when you've been working on TerraGear, but a
> lot has changed in the meantime. Anyhow, GRASS doesn't work wonders,
> the garbage-in - garbage-out principle still applies. That's why I've
> been working on creating clean datasets .... which has the neat
> side-effect that you don't need to have a "landmass" any more.
>
> > And you could create a topologically clean land cover database which is a
> > good thing. But then try to mix this with some other dataset (airports
> > from Robin for instance) and be right back to having topological problems
> > again.
>
> No :-)
>
> What I was having in mind - and what I actually did - is converting the
> airport holes (the work directories written by "genapts") into shapes
> and cutting these out of the land cover. Voila, topologically clean
> airport hole boundaries (I admit you have to dabble with the Z axis a
> bit).
> I've done the same with OSM road data - look at this sample, these
> roads are real polygons and I've been cutting these out _before_
> feeding the result to TerraGear:
>
>
> http://mapserver.flightgear.org/map/?lon=6.5045&lat=51.23036&zoom=14&layers=0000000BTFFFFTFFTFFFFFFF
>
> That's how the result looks like (a screenshot I made for a
> presentation at PGConf.DE last year):
>
> http://foxtrot.mgras.net/bitmap/FGFS/EDLN-OpenLayers-RoadCutout.png
>
> Cheers,
> Martin.
> --
> Unix _IS_ user friendly - it's just selective about who its friends are !
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