[GRASS-user] v.generalize for polygons?

[Hamish]

Hamish:
> > I have a high-res vector area map of regional districts which I
> > wish to generalize. I am having trouble with finding the correct
> > method in v.generalize to use. Currently every thing I try tends
> > to break the area topology and leave only a portion of the now-
> > open boundary.

I have now tried with a related vector, linked below, and it worked
(very!) nicely for that. But it fails with a derivative vector map.
v.digit shows no problems with topography.

Wolf:
> What methods did you try?

many of them.. mainly douglas with a number of threshold values.

> What exact commands have you tried that fail? 

at the simplest:   v.generalize in= out=
but some areas are missing.


> Can you share the problematic map? (you can email it to me directly)

sure,

starting with:
http://www.stats.govt.nz/statistics-by-area/regional-statistics/geography-mapping/download-digital-boundaries.htm
-- Census based NZMG 2006 (37mb shapefile .zip)

I am looking at regional boundaries (RC) from REGC06_LV2.shp

this map generalizes nicely, but it includes the 12 nautical mile
territorial buffer around the coastline. When I overlay that map with a
detailed coastline is when I see the problem.

I'll send a sample of the v.overlay output off-list.


> v.generalize does preserve nodes, and as long as the input map is 
> topologically correct so should the output map be.

ok. (confirmed, it does a very nice job simplifying the above
shapefile)

> Perhaps your threshold is way off?

Possible, as I am just learning. But I did try a number of ranges and
slowly increase. All would be ok for slight generalization then big
breakage.

e.g. it has a big jump between thresh=0.4865 and 0.487

Good:
   v.generalize in=rc_merge_coast3 out=rc_gen thresh=0.4865 --o
   ...
   Number of vertices was reduced from 569815 to 521969 [91%]

Bad:
   v.generalize in=rc_merge_coast3 out=rc_gen thresh=0.487 --o
   ...
   Number of vertices was reduced from 569815 to 336380 [59%]


Daniel:
> However, there is a flag(-r?) which prevents the module from removing
> them.

Flags:
  -c   Copy attributes
  -r   Remove lines and areas smaller than threshold



thanks,
Hamish




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