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

[Hamish]

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