[GRASS-user] creating new geometry from pre-existing lines and points

[Nikos Alexandris]

On Thu, 2008-01-31 at 09:00 +0200, Maris Nartiss wrote:
> I'm sorry to interrupt Your advanced discussion, but if there are just
> some "holes" that are required to be converted to polygons, then why
> not use v.digit? Simply digitising new centroid inside a hole
> (enclosed from other polygon boundaries) will turn hole into polygon.

Yes,

it is simple(r). And there is no need to "save as shapefile" and
re-import in GRASS the modified vector file.

> 
> WBR,
> Maris.
> 
> 2008/1/31, Nikos Alexandris <nikos.alexandris at felis.uni-freiburg.de>:
> > Hi!
> >
> > On Wed, 2008-01-30 at 23:52 +0200, David Epstein wrote:
> > > Hi Nikos,
> > >
> > > Thank you for that suggestion. Indeed, I did NOT realize that v.extract
> > > was used to create new geometry in a layer. However, I too need some of
> > > the islands (is there a technical difference between island, hole, and
> > > missing polygon?).
> >
> > I am not sure!
> >
> > Would be nice to have a kind of GRASS-GIS lexicon with (basic and
> > non-basic) definitions!
> >
> > * island... I suppose are small poylgons within larger ones
> >
> > * holes... just "nothing" within larger polygons
> >
> > * missing polygon... ?
> >
> > >
> > > There is a single missing polygon that I now have attribute data for and
> > > I want to create the polygon (from the existing points and lines) and
> > > fill-in the related attribute fields.
> > >
> > > I imagined some sort of interactive process of clicking on the relevant
> > > bounding elements and *presto* the polygon would jump into existence. I
> > > tried this in v.digit but was not able to get the desired result.
> >
> > If you just need to quickly correct a small number of polygons try with
> > the "fresh-baked" QGIS (r0.9.2.rc1), which works as a GRASS frontend.
> >
> > 1. you need to add in your /etc/apt/sources.list following source:
> > deb http://ppa.launchpad.net/timlinux/ubuntu gutsy main
> >
> > 2. sudo aptitude update && sudo aptitude upgrade
> >
> > --- if not already installed... "sudo apt-get install qgis
> > qgis-plugin-grass"---
> >
> > 3. Load GRASS plugin
> >
> > 4. Open your GRASS mapset within QGIS
> >
> > 5. Load your vector (polygon) file
> >
> > 6. Play with the tools... ( I have seen the new digitising tools... must
> > be really what you are looking for!) and edit your polygons.
> >
> > *** You need first to "right click" on your vector layer and "save as
> > shapefile" -- Then you can edit.

7. Re-import in GRASS with v.in.ogr
> >
> >
> >
> > >
> > > This is a really basic step that people do several times a day, right?
> > >
> > > -david
> >
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-- 
Nikos Alexandris
.
Department of Remote Sensing & Landscape Information Systems
Faculty of Forestry & Environmental Sciences, Albert-Ludwigs-University Freiburg
.
Tel.  +49 (0) 761 203 3697 / Fax.  +49 (0) 761 203 3701 / Skype: Nikos.Alexandris
.
Address: Tennenbacher str. 4, D-79106 Freiburg i. Br., Germany
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