[GRASS-dev] v.build: why is attaching islands taking so long?

[Hamish]

Maciej Sieczka wrote:
> >> Can anybody possibly explain why does the "Attaching islands..." in
> >>  v.build take so very long compared to other operations? For
> >> example: I have a big vector map consisted only of lines, more than
> >>  180 000 of them. v.build on this dataset takes about 20 seconds.
> >> After converting lines into boundaries (v.type), v.build on that
> >> new vector map made of boundaries only takes half a day, with 99%
> >> of processing time dedicated to "Attaching islands...". Is that
> >> normal? Any improvements possible?

Hamish:
> > the not-thinking-about-it-much guess is to blame the "florida
> > coastline" problem. to test is that is the problem, try running
> > 'v.split vert=1000' before v.type and see if it still takes a long
> > time.
> 
> 'v.split vert=1000' reduces the processing time from over 10 hours to
> 10 minutes. Strange world we live in.
> 
> > you might try running a subset through a profiling tool if you like:
> http://grass.osgeo.org/wiki/GRASS_Debugging#Using_a_profiling_tool
> 
> If I find time I'll try and post here.

not really required to run the profiler now we know what the problem 
probably is. from Radim's
  http://trac.osgeo.org/grass/browser/grass/trunk/doc/vector/TODO#L241

"v.in.ogr
--------
It would be useful to split long boundaries to smaller
pieces. Otherwise cleaning process can become very slow because
bounding box of long boundaries can overlap large part of the map (for
example outline around all areas) and cleaning process is checking
intersection with all boundaries falling in the bounding box."


see
 http://article.gmane.org/gmane.comp.gis.grass.devel/7400
   mailing list link there needs to be adjusted by +193, now:
 http://lists.osgeo.org/pipermail/grass-user/2005-April/028711.html
   and
 http://intevation.de/rt/webrt?serial_num=3161



Hamish