[GRASS5] v.digit topology question

Radim Blazek blazek at itc.it
Thu Oct 30 03:41:51 EST 2003

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.

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

> I've tried inserting a centroid, doing various versions of v.clean,
> v.build.polylines, and v.build to no avail. After exporting into ascii,
> cleaning up some tiny segments produced at line intersections (how
> produced I don't know, perhaps by v.clean), and reimporting, I managed
> to get it down to a single polyline boundary. But I still can't convert
> it to an area.

With v.digit, it should be possible to create clean data. To make clean
data in v.digit is better than to make bad data and run v.clean later.

> 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.

> On Wednesday, October 29, 2003, at 10:01 PM, John Gillette wrote:
> > v.type input=map_in output=map_out type=line,boundary
> Thanks for the advice. I thought about this and almost tried it but did
> not. The reason is that GRASS considers my closed polygon object to be
> a boundary, but not an area. I want a filled area, not just the
> boundary of an area. v.type does not have an option to make a boundary
> an area.

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


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