[GRASS-user] Extracting the external boundary of a vector file

Edward Tomlinson e.j.tommo at gmail.com
Thu May 29 05:38:30 EDT 2008

Hi Hamish,

Thank you for the suggestions. v.hull is something that i had passed over
before, it wont work on this occasion as the dam is boomerang shaped, but i
will be sure to use it when i can. Takes a fair deal of processing out.

For those that are interested the final result was as follows:
v.category option=add cat=1 step=0 type=area (this filled in my islands - as
they weren't islands, just obsticales)
v.extract type=area new=1 list=1-99999
2008/5/29 Hamish <hamish_b at yahoo.com>:

> Edward Tomlinson:
> > I have a whole bunch of data from an RTK GPS. File exists
> > as a MapInfo file.
> > Imports into grass fine with area and centroids.
> >
> > My problem is however, i want to create another vector file
> > that contains the mask or the external (an internal island) boundary.
> >
> > I have been able to create the external boundary fine
> > through the following.
> > v.category (delete categories)
> > v.category (add category as 1 for all)
> > v.dissolve (combine all boundaries with common category)
> >
> > This is great and works fine for the external boundary.
> > However there is meant to be an island in there. Somewhere
> > along the way (i think after the second call to v.category)
> > it fills that island in.
> right.
> > Has anyone got any hints or ideas?
> >
> > Essentially i am trying to create this vector to generate a
> > mask for further interpolation of the point heights. I am
> > assuming that i need this island removed (as there is no
> > point data for it) otherwise it is going to effect
> > the interpolation.
> Use v.overlay for the new and original map, operator=not, etc.
> or other combination of operator=and etc until you get what you want.
> another trick is "v.extract list=1-99999999" to only extract features with
> a cat number. (then v.overlay)

> v.hull is another module to look at for this instead of v.dissolve.
> Hamish
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