[GRASS-user] Vector Topology: how to get a line from a boundary

[Maciej Sieczka]

peter.loewe at gmx.de pisze:

> 1) How to get "just the outline" of the island ? Both v.type and 
> v.extract with "type=boundary,line" produce an empty vector.

The manual reads:

---
NOTES

Only features with a category number will be extracted. So if you want
to extract boundaries (which are usually without category, as that
information is normally held in the area's centroid) you must first use
v.category to add them.
---

> 2) The main issue is to derive all counties of the island which lie 
> on the shore (= ignore the counties which don't have a beach). How 
> can a topological query be set up, saying "derive all the county 
> polygons which share a common boundary with the islands shoreline" ?

You might try v.to.db opt=sides col=left,right. After that select those
areas, of which at least one boundary only has 'left' or 'right' populated.

Maciek

-- 
Maciej Sieczka
www.sieczka.org