[GRASS-user] Removing detail from polygons

[Markus Neteler]

On Thu, Jul 27, 2006 at 09:34:16AM +1200, Brent Wood wrote:
> Jose Gomez-Dans wrote:
> >Hi!
> >I have a large shapefile with polygons. It takes a long time to load
> >it into GRASS with ogr, but my question is other. I only need to use
> >this polygon for drawing "informal" maps (i.e., they don't need to be
> >accurate). The detail of these polygons is huge, so that if I use plot
> >them with GMT or any other tool, I get massive EPS files. I don't need
> >most of those vetices. Ideally, I'd like to specify some distance so
> >that I get a single line, rather than a myriad lines. I thought
> >v.clean could do this, but I am unsure whether it is the right tool or
> >not.
> >
> 
> I've used PostGIS for manah\ging spatial data. It is possibly overkill 
> for what you are asking, but using the shp2pgsql program that come with 
> it, you can easily load a shapefile into PostGIS. You can then easily 
> simplify the geometry & use pgsql2shp to export the new version. I have 
> used it for making scale dependent layers of the same data for plotting 
> purposes, nmuch like you seem to be requiring.
> 
> The PosGIS simplify() function is an implementation of the 
> Peuker-Douglas algorithm which is commonly used for this purpose. IT is 
> quite handy, in that despite the function operating in projection units, 
> you can wrap it in transform(), so you can take a lat/long geometry, 
> transform to an appropriate meter based equidistant projection, simplify 
> & project back again in a single SQL statement.
> 
> ie: transform(simplify(transform(geometry)))
> 

Hi Brent,

while I would love to see Douglas-Peucker implemented in v.clean
or a v.generalize (the GMT site has a C implementation of it),
I wonder about simply features versus topology issues:

http://mpa.itc.it/markus/como2006/img8.html
-> done in PostGIS (freegis.org world map simplified from VMAP0)

http://mpa.itc.it/markus/como2006/img9.html
-> done in GRASS

The latter looks better to me.

Markus