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

[Moritz Lennert]

Jumping into the debate, since it raises some fundamental questions in 
my eyes;

On 19/04/10 17:46, G. Allegri wrote:
> No problem, this is not what I expect. I just expect a user to be able
> to import a polygonal layer, without worrying about topology correctness
> (clean/build operations), and do spatial operations on it. Obviously the
> correctness of results depend on the operation the user is doing (and
> it's his problem) but, i.e. a geometrical clip will be always correct, I
> think...

The questions here for me are:


- What is the actual error linked to doing operations on non-topological 
formats ?

- Why do you absolutely want GRASS to do this ? If all you want to do is 
simple geometry operations on spaghetti files, you can easily do this 
with most of the available open source viewers / GIS, such as QGIS, 
gvSIG, uDIG, etc or even directly with ogr. Why go through the trouble 
of using GRASS for that ?

>> These operations should be quite fast for native vectors with topology.
>
> With "rapidly" I meant: load data, do-the-op, save the results. I'm sure
> that the native grass data structures can deal more efficiently then SF
> structures.... but often a user prefer to wait a minute more for the
> operation to end, then working a minute to have to manage the data
> cleaness (more often it takes much more then a minute, and it doesn't
> worth it!)
>
>  > Yes, although not that rapidly because pseudo-topology needs to be
> built first. Polygons do not need to be clean, but then nobody can
> guarantee for the results.
>
> Ok, but the user just have to wait and wath the progress bar
> completing... that's ok from his perspective :)

I find it a bit suspicious when user comfort is apparently put so much 
higher in priority then correctness of results...

Moritz