[GRASS-user] v.overlay missing polygon

[Vincent Bain]

Never tried this.
I am generally very cautious with snapping. Not knowing exactly how
grass handles it but one should be aware of possible topological
alterations, duplicate features, etc.

Vincent

> 
> Dear Vincent,
> 
> I was able to reproduce your code. This is definitely an improvement! 
> For the test data this solution works.
> I applied the snapping code to a larger data set. There was only (or 
> still) one case left where a polygon dissapeared. I guess the reason 
> was, that there was a problematic overlap but no vertices within the 
> snapping threshold. In this case it would be desirable that snapping 
> could also be done with boundaries.
> 
> I think a snapping option in the manner of your suggestion should be an 
> option in the v.overlay and v.patch tools. Disappearing polygons are 
> really problematic for my work and this could be a solution.
> 
> Actually a friend told me that in PostGIS there is an option to snap all 
> vector data to a grid. This would be possible in Grass if you would 
> produce a vector grid (maybe from a raster file with an appropriate 
> resolution) and use it as a bgmap. Did you ever try this?
> 
> Falko
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