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

[G. Allegri]

Markus, I do not doubt that grass data sructures permits efficency and
rigorous processing. And I haven't your knowladge and experience on
this field. I'm just putting myself on the (lazy?) end user experience
side, and from this point of view there's a foundamental difference
between grass and others (like saga): the v.clean step. Two recent
cases we had:

1 - we had to make a simple points in polygon count. The polygon layer
wasn't 'clean' (we hadn't perfect boundary adjacency for example), and
it was made of hundreds of thousands of polygons. The v.in.ogr
process, and the necessary clean, was taking too much time respect to
the simple operation we needed, so we imported it in saga, and with a
few clicks we had our result.

2 - we had to create a single area from a parcel shapefile, made by
lines. We could do it only with grass, thanks to its topology, through
v.type, and this worths the cleaning step...

So, I know that grass lets me to do a lot more then "the others". In
synthesis, I was just wondering if for processing ops like in case 1
one could avoid the v.clean phase...


2010/4/20 Markus Metz <markus.metz.giswork at googlemail.com>:
>
>
> 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
>