[GRASS-user] Problem with v.generalize to simplify areas

[Markus Metz]

v.generalize does a great job with simplifying or smoothing lines, but 
areas either disappear or attribute attachment changes. That's why I 
would recommend not to use v.generalize for areas. v.clean tool=prune 
behaves the other way around, areas are always preserved, with attribute 
attachment, as they are, but line simplification does not always work, 
i.e. produces strange results or does not simplify at all. Should we 
swap the warnings, e.g. say for v.generalize that there are problems 
with processing areas and for areas use v.clean tool=prune instead?

Markus M


Marc-Antoine Nussli wrote:
> Hi,
>
> I'm trying to simplify a vector map of areas describing countries.
> I tried to use v.generalize with the various proposed simplification 
> method but the result is completely wrong as soon as I set a high 
> threshold value(actually, it works only with very low values) The 
> resulting map is always completely broken with most of the areas 
> having disappeared.
>
> I have attached a sample file(in grass standard ascii format). If you 
> run v.generalize with a threshold of 100, the map will be broken but 
> with a value of 10, it works. Note that a threshold of 100 is still 
> very low considering the resolution of the map...
>
> I know that there has been a few posts on this topic but I could not 
> find a answer which corrects the problem.
>
>
> Interestingly, I tried instead to use v.clean with the "prune" tool 
> and the result is very good. It works with very high threshold (at 
> least up to 10000) but I don't know if I can trust it as it is written 
> in the documentation that "The /prune/ tool is currently broken, 
> please use /v.generalize 
> <http://grass.osgeo.org/grass64/manuals/html64_user/v.generalize.html>/ 
> instead." So, it is a bit contradictory with the results I got?
>
> Do you know why v.generalize doesn't work with high threshold?
> And do you know what is the actual status of the "prune" tool of v.clean?
>
> I'm using grass 6.4RC5 on linux.
>
> Thanks!
>
> ma
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