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

GRASS GRASS user
1

Hi,

here is a vector topology related issue:

After merging a base layer of adjacent polygons ("all the counties of an island") into one polygon ("the island's polygon") I am riddled by two issues:

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

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" ?

Peter

--
Dr. Peter Löwe
<peter.loewe@gmx.de>

Psssst! Schon vom neuen GMX MultiMessenger gehört?
Der kann`s mit allen: http://www.gmx.net/de/go/multimessenger

2

peter.loewe@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

3

Hi,

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.
---

you should be to extract boundaries without category by setting
'layer=-1' , e.g.

v.extract in=landcover out=b type=boundary layer=-1 new=-1

Martin

--
Martin Landa <landa.martin gmail.com> * http://gama.fsv.cvut.cz/~landa *