[GRASS5] v.out.ogr question

[Radim Blazek]

On Thursday 18 March 2004 10:59, Otto Dassau wrote:
> > >  |   Number of points:       0               Number of areas:      1   
> > >  |  | Number of lines:        0               Number of islands:    1  
> > >  |   | Number of boundaries:   1               Number of faces:      0 
> > >  |    | Number of centroids:    1               Number of kernels:    0
> > >  |     | Number of dblinks:      1                                     
> > >  |      |
> > >
> > > GRASS 5.7.-cvs:~/test > v.out.ogr ammprv dsn=ammprv layer=ammprv
> > > WARNING: Requested types: points and lines, but (additionally) areas
> > > found. Check 'type' parameter.
> > > 1 features written
> > > WARNING: 1 features without category written
> > >
> > > -> Strange: I would expect 0 features written.
> >
> > Why strange? Requested type is 'line,boundary' (default) and there is 1
> > boundary in ammprv and that boundary is exported.
>
> Hi Radim,
>
> I simply think it would be nice, if v.out.ogr doesn't export any data,
> as long as the parameter 'type' is not set correctly. This would just make sure,
> that the user really gets what he intends. I know it is not an error, more a whish and
> maybe also too petty.

This particular case is not important, but this issue in general is very important.
One of very few advantages of GRASS over proprietary competitors is that it does what
user requested specifying parameters and it does not do what maybe could be
intendet result of majority of users.
With GRASS, I am sure that I can do what I want, and I cannot, for example, 
end up with with impossibility to export boundaries as lines to shape only 
because users mostly exports areas and they don't read option descriptions.

Back to the case above. How can the module recognize that you don't want 
boundaries and you have done a mistake? 

Two examples when what v.out.ogr does in this case is correct:
1) transfer data to another topological GIS, that means we want boundaries
   as lines, not closed polygons
2) 1000 vectors in GRASS, not all of them contains all possible features,
   if those vectors are exported (srcipt), even if no requested feature is present,
   correct and desired output is empty file, not missing file.

Radim