[Qgis-user] Fwd: Merging polygons leaves isolated lines

[Lynkos]

I've been given a shape file representing a vegetation map which I now have
to modify, merging a number of polygons. When I do this, in almost all
cases the merge leaves a number of lines, either attached to one of the
polygons or completely isolated. An example of the problem (one of the
worst) is illustrated in the following screenshots.

Firstly a section of the shape:
https://www.dropbox.com/s/amik6g9cdoqg84b/Screenshot%202014-01-15%2017.42.18.jpg

The three polygons to be merged selected:
https://www.dropbox.com/s/6gl83y2jwgxvrje/Screenshot%202014-01-15%2017.42.33.jpg

The section of the shape after the merge:
https://www.dropbox.com/s/f1iem85kidkwkwx/Screenshot%202014-01-15%2017.42.53.jpg

And a close-up of the unattached lines left after the operation:
https://www.dropbox.com/s/i4rmyi7apcfpgg9/Screenshot%202014-01-15%2017.44.05.jpg

In case it is relevant, here are my snap options:
https://www.dropbox.com/s/73ky3qgbzk16esj/Screenshot%202014-01-15%2017.44.30.jpg

And here is a part of the shape file:
https://www.dropbox.com/s/n2asd9urvyd5fc8/merge-test.shp

If I had just a few polygons to merge, I could simply eliminate the lines,
but the file contains about 500, a large proportion of which must be
merged. I imagine the problem is caused by errors in the original polygons,
but the errors don't show up on either visual examination or using the
verify validity function.
Any ideas on what the problem might be and how to resolve it, if possible,
eliminating it at the roots by correcting the original shape?
Thanks, Sarah.

P.S. Let me know if you have problems visualizing the images or downloading
the file.


-- 
*Sarah Gregg*
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