[GRASS-user] [newbie] How to split an area with v.digit?

GRASS GRASS user
1

Hi list,

I have used v.digit to define a closed area with several border vertices and a centroid.

The resulting v.info shows:

Number of points: 0 Number of areas: 1
Number of lines: 0 Number of islands: 1
Number of boundaries: 1 Number of faces: 0
Number of centroids: 1 Number of kernels: 0

Now I want to subdivide the area into two parts. How can I do this?

I tried to restart v.digit and digitize a new boundary, starting and ending at existing border vertices.
Unfortunately, the resulting border line is not connected to the border: It stays gray like a boundary that does not limit an area, and does not become green like a boundary between two areas.
Zooming in very close shows that the start and end points of the new boundary have not snapped (and do not snap) to the existing border vertices :frowning:

Thank you for any hints,
Robert

--
______________________________________
Dipl.-Ing. Robert Pollak
SCIETEC Flussmanagement GmbH
Herrenstraße 4
A-4020 Linz

Phone: [+43] (0)732 / 71 31 20 26
Fax: [+43] (0)732 / 71 31 20 4
Cell Phone: [+43] (0)699 / 11 44 30 32

Mailto: robert.pollak at scietec.at
URL: http://www.scietec.at
______________________________________

2

On Wed, Dec 06, 2006 at 02:13:40PM +0100, we recorded a bogon-computron collision of the <robert.pollak@scietec.at> flavor, containing:

Hi list,

I have used v.digit to define a closed area with several border vertices
and a centroid.

[...]

Now I want to subdivide the area into two parts. How can I do this?

I would like to know, too.

I tried to restart v.digit and digitize a new boundary, starting and
ending at existing border vertices.
Unfortunately, the resulting border line is not connected to the border:
It stays gray like a boundary that does not limit an area, and does not
become green like a boundary between two areas.

Are you starting and ending at border *nodes* or just vertices? v.digit
doesn't snap new line nodes to vertices on an existing line, only nodes at
the ends, as far as I can tell.

What I've seen to be necessary is to *split* the existing boundary at the point
where you want to make the new connection so that there is a node there.
(there is a split-line tool in the v.digit palette) Then when you try to
create the new boundary segment starting there it will snap the new start
point to the existing node.

It would be nice if there were a way to combine the two operations --- that
is, unlike the existing line tool, if we click within snapping distance of
an existing line then the existing line is broken at the intersection.

Near as I can tell, one can get close to this by digitizing the first areas,
then digitizing the splitting lines so that they intersect the exisitng
boundaries, then using v.clean to break lines at intersections and remove
dangles. But if you have digitizing errors such that the new line has a gap
between its end and the existing line instead of a dangle and an intersection,
this doesn't get fixed by v.clean (as far as I can tell).

Is there a simpler way?

I have an pipe dream application for GRASS that is still a pipe dream because
of exactly the difficulty in creating subdivided areas easily in the digitizer.

--
Tom Russo KM5VY SAR502 DM64ux http://www.swcp.com/~russo/
Tijeras, NM QRPL#1592 K2#398 SOC#236 AHTB#1 http://kevan.org/brain.cgi?DDTNM
"And, isn't sanity really just a one-trick pony anyway? I mean all you get is
one trick, rational thinking, but when you're good and crazy, oooh, oooh,
oooh, the sky is the limit!" --- The Tick

3

Tom Russo schrieb:

I have used v.digit to define a closed area with several border vertices and a centroid.
Now I want to subdivide the area into two parts. How can I do this?

[...]

Near as I can tell, one can get close to this by digitizing the first areas,
then digitizing the splitting lines so that they intersect the exisitng boundaries, then using v.clean to break lines at intersections and remove
dangles.

Thank you - this is cubersome, but working.
I am defining an intersecting border line, then I use

GRASS 6.0.1 (spearfish60):~ > v.clean input=map1 output=map2 tool=break,rmdangle thresh=0,30

-- Robert