Hi Martin,
Ok, understood. I just wanted to make sure what I imagined "topologically
clean" might mean matched your usage. :-)
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.
This can lead to any number of problems -- ranging from crashing GPC itself
to crashing just about any of the down stream code.
I've spent hundreds of hours (probably) investigating individual cases and
crafting detection/fixing code for a variety of types of problems. But as
you increase the resolution of the data, you just increase the number of
these problems, probably stack them on top of each other in some cases, and
probably invent some new ones I hadn't seen in the lower res data.
Well, it's just a whole lot of fun. :-)
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.
I have been wondering if some sort of rasterized painting algorithm would
be simpler and more robust (at the expense of some resolution.) And this
could be done "offline" and not involve opengl or rendering at all -- just
big 2D arrays. Essentially paint a big image from bottom to top using your
polygon data, and then extract the result into actual polygons -- kind of
the same process as we do now, but do it in raster space rather than vector
space. Every once in a while I've wondered if we've made the problem too
hard for ourselves and maybe we should be seeking a bit simpler solution
and learn to live with some different set of trade offs?
Best regards,
Curt.
On Mar 13, 2012 8:26 AM, "Martin Spott" <martin.sp...@mgras.net> wrote:
> Curtis Olson wrote:
>
> > Just a dumb question: can you give a brief summary of what "topologically
> > clean" means?
>
> I guess you always wondered why I've been pounding on these datasets
> for so long ;-)
>
> Martin.
> --
> Unix _IS_ user friendly - it's just selective about who its friends are !
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