[GRASS5] Re: [5.7] remove islands and U-dangles?

[Hamish]

> > > > Also, 'v.extract type=area list=1,2,3' keeps areas with missing
> > > > centroids; how do you automatically add missing centroids so you
> > > > can then get rid of the zombie areas with another [xxxxxxxx]?
> >
> > => 'v.extract -d list=1-50000'
> >
> > I thought 'v.clean tool=bpol' would do this, but it doesn't.
> > Should it? Or should/is this better in another tool?
> 
> So again, any space closed by boundary, even if it does not have any
> centroid is area. It is area without centroid, without category. 
> We can discuss, if d.vect should (and when) display those areas, se
> more below. It is not a bug! It is not a bug! It is not a bug!

Can those lines/boundaries be picked by using 'v.select list=0' ?
Would/could that work?


> In QGIS, one field means layer. If an area in GRASS has a category of 
> field 1, it is represented as feature in the layer polygon_1.
> But it is different from GRASS, because user can see in the dialog,
> that there is also layer polygon_0 (islands without category)
> and he can add it to the map as another layer. 
>
> I don't know if it is good or not, we have to test it,
> but it seems that it is more suitable in most cases and for
> most users. 
> 
> My suggestion is to leave d.vect and d.what.vect as it is 
> with all the complexity for experts and follow layer approach
> (as described above) in user-friendly viewers.

yes.

> I would appreciate, if you could try QGIS and tell me if it
> does what you expect. If we find the QGIS approach good, the 
> same will be used for GRASS driver in OGR.

Ok, got it to compile after forcing the configure script to look in
/usr/include/qt3  [needed the libqt3-mt-dev debian package].

After installing, and starting qgis from within GRASS 5.7, I could plot
up the polygon_0 layer to highlight where the areas without centroids
that I needed to get rid of were (line legend -> red,50 wide). Then,
knowing where they were, I could go back to v.digit and remove them.
Success!


Two QGIS glitches (latest qgis CVS):
a) when zooming in a moderate amount (with the polygon_1 enabled) QGIS
would freeze my X-windows for 2-3 minutes at a time. Mouse worked,
everything else was frozen. After the wait it would redraw. Had to kill -9
it remotely a couple of times..

b) Some of the polygon_0 areas wouldn't show up. With a huge line width,
some would show up as a vertical bar, others blank. By playing around
with clicking on values in the attribute table to highlight the
individual polygons and changing the search radius for querying
by mouse [blind], I was finally able to track down the last few by
brute force. Yellow on white highlight for the attr table is really hard
to see..


Who is the polygon_2 layer and what does it do?



cheers,
Hamish