[GRASS-dev] v.external, ogr direct access, topology and pseudo-topology.... a bit confused

[Markus Metz]

On Tue, Apr 20, 2010 at 10:25 AM, G. Allegri <giohappy at gmail.com> wrote:

> > But even a simple overlay can give wrong results if the data is not
> > topologically clean... I personally cherish the fact that GRASS doesn't
> make
> > it easy to do things quick and dirty.
>
> Probably we mean different things, or maybe I've msunderstood topology
> in Grass. Spatial operators like Overlay generally leaverages on
> topology graph structures (like in GEOS), but I suppsed that
> pseudo-topology (as meant in Grass) would be enough to guarantee the
> results. Isn't it?
>

Most GRASS modules require information available in topology which might be
true vector topology or other auxiliary information. This other auxiliary
information also helps to make operations faster and safer, and BTW this is
not a peculiarity of GRASS to require some auxiliary information. Reducing
this info is in most cases not an option because it would sacrifice the
accuracy of results. It remains unclear to me why you insist on reducing
topological (true topological and other auxiliary) information gathering.
Correct me if I'm wrong but AFAICT in GRASS there are equivalents to the SF
algorithms you have in mind, IOW, in GRASS you can do everything you can do
in e.g. SAGA, and a (fair) bit more. Many modules and functions already
reduce the amount of topological information to the minimum required for the
current operation, e.g. v.clean.

Replying to your next mail, both pseudo-topology and full topology are at
feature level for all layers in a vector map.

Markus M
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