[GRASS5] v.digit topology question

Michael Barton michael.barton at asu.edu
Thu Oct 30 04:21:26 EST 2003


Thanks much for the information.

On Thursday, October 30, 2003, at 09:41 AM, Radim Blazek wrote:

> On Wednesday 29 October 2003 21:10, you wrote:
>> However, something is driving me crazy. I've probably missed 
>> something,
>> but hope someone can help me. I've digitized a boundary, but can't
>> figure out how to turn it into an area. When I do a right-click after
>> digitizing a shape, the implication is that it will close a polygon,
>> creating an area. It doesn't seem to do this,however.
> Right-click does not close the polygon (yes, 'Close line' prompt is 
> not the best).
> To do this, new check box ('Close polygon') must be added to interface.
> I want to add also option 'Insert automatically new centroid'.
> These two options should make digitizing of isolated areas easier.
> Now, if you want to create new area as one closed boundary, you have 
> to digitize
> the boundary and then to snap last vertex to the first vertex.
> Boundary and crosses at the ends must become green not red.

After considerable futzing, that is where I am at now. When I first 
digitized this, it happened to be in several separate segments (green 
"x" at intersections). I eventually ran v.clean, separating all 
segments, followed by v.build.polylines to put them together into a 
single boundary (one red "x" at the closing intersection). However, I 
had to do a v.out.ascii, edit the ascii file to get rid of some tiny 
segments, followed by a v.in.ascii to finally get to a single boundary 

> BTW, there is a bug in vector library and centroid colors are not 
> updated
> properly if boundary is closed, opened etc.

This may have caused me to think I had a bad polygon when I was 
actually OK. However, I regularly ran v.build to look at the messages 
to see if any centroid I entered actually got linked up to a boundary. 
In all cases, it was seen by v.build as an isolated centroid (i.e., 

>> Also, please note that when I do a v.out.ascii on this boundary file,
>> it creates a file that GRASS 5.0.x won't parse. I assume that it has
>> something to do with assigning "B" labels for boundary vertices, 
>> rather
>> than "A" or "L" labels, but I don't know for sure. I haven't yet tried
>> exporting in version 4 format. Maybe that will work.
> Vector ascii format is changed, use -o (old) option to get 5.0 format.

That was what I eventually figured out. The message on the TCL dialog 
box for v.out.ascii describes the -o option as for version 4.

> Most probably, the polygon is not closed. What color is the boundary 
> in v.digit?

You could well be right. The polygon is red. Once I managed to get it 
to orange but still couldn't get the centroid to link up (or perhaps 
this was where it actually was linked but I didn't know it because of 
the color update bug).

I have moved and resnapped the polygon closing ends repeatedly, zooming 
in to as close as I can. I've also tried to adjust the snapping to no 
avail. I just tried it again, adjusting the snapping to 5m and zooming 
in to a very close range to reclose the polygon. I then put a centroid 
in the polygon and exited.

The polygon still doesn't show up as a filled area.

If it helps figure out what may be going on, nothing happens when I 
click the show attributes button and select either the polygon or a 
centroid I created.

C. Michael Barton, Professor
Department of Anthropology
PO Box 872402
Arizona State University
Tempe, AZ  85287-2402

Phone: 480-965-6262
Fax: 480-965-7671

More information about the grass-dev mailing list