[GRASS-dev] [bug #2765] (grass) v.buffer produces strange results

Maciek Sieczka via RT grass-bugs at intevation.de
Tue Nov 21 15:30:19 EST 2006

Hamish (and All),

I have revisited this bug and I think I know why you, after your fixes, were
able to produce correct buffers using my 'ditches' vector, while I was still
obtaining errors (though somewhat different than at the beginning, before your


1. unpack the bzipped testing location I have sent you

2. enter the location - don't touch anything yet!

3. v.buffer input=ditches output=buf1 type=area buffer=1

4. display 'buf1'; you should see the wrong buffer around the parcel that has
cat 1205 in the 'ditches' vector - as I did

5. now, open 'ditches' in v.digit, pan to where cat 1205 is and disjoin it's
boundary, and the snap the vertiices back as they were originally

6. v.buffer input=ditches output=buf1_new type=area buffer=1

'buf1_new' is OK! - although the command in point 3. and 5. is identical, and
although the input data haven't changed a bit - only one vertex was moved, but
snapped back to it's original position immadietly.

I have no idea why it is like that. Hamish, can you reproduce that? Does this
explain how the same data worked for you but not for me?

Strange. The input vector in both cases is virtually the same, in both it is
topologicall, error free (according to v.build, v.clean and visuall inspection
in v.digit). But somehow, moving one vertex slightly and snapping it back
"fixes" the vector so that v.buffer can handle it OK.

'ditches' was hand digitised by me in QGIS 0.74, few months ago.


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