[GRASS-user] v.generalize issues

[Markus Metz]

On Thu, Jun 30, 2011 at 7:49 PM, Christian Guirreri <guirreri at gmail.com> wrote:
> I suppose my other question is if anyone has any recommendations concerning
> v.generalize as it applies specifically to tiger data - I'm doing all of US.
> Lots of settings and tests and its a bit overwhelming for a newbie.
>
For simplification (vertex reduction), Douglas-Peucker is generally a
good choice, it's relatively simple and commonly used.

For smoothing, Chaiken's Algorithm and Hermite Interpolation should
produce quite different results, apart from being a matter of taste,
it depends on what kind of modifications are allowed.

Markus M


> On Jun 30, 2011 11:33 AM, "Christian Guirreri" <christian at guirreri.com>
> wrote:
>> Success! Thanks for making this easy for the total noob that I am.
>>
>> On Thu, Jun 30, 2011 at 11:04 AM, Markus Metz <
>> markus.metz.giswork at googlemail.com> wrote:
>>
>>> On Thu, Jun 30, 2011 at 4:47 PM, Christian Guirreri
>>> <christian at guirreri.com> wrote:
>>> > I'm on GRASS 6.4.1. QGIS 1.7.0
>>> >
>>> In this version, v.generalize is still broken. You would need a recent
>>> version of GRASS 6.4.2. For Windows, you can get a recent version
>>> here:
>>>
>>> http://wingrass.fsv.cvut.cz/grass64/
>>>
>>> Markus M
>>>
>>>
>>> > On Thu, Jun 30, 2011 at 10:18 AM, Markus Metz
>>> > <markus.metz.giswork at googlemail.com> wrote:
>>> >>
>>> >> On Thu, Jun 30, 2011 at 4:01 PM, Christian Guirreri
>>> >> <christian at guirreri.com> wrote:
>>> >> > I'm a brand new user to GIS - my goal right now is to heavily
>>> >> > simplify
>>> >> > Tiger
>>> >> > 2010 county and distrct data, without producing gaps between
>>> boundaries.
>>> >> > I've been testing this with v.generalize in grass via QuantumGIS -
>>> I've
>>> >> > had
>>> >> > tons of issues with the Grass toolbox crashing, but have narrowed
>>> >> > down
>>> >> > to
>>> >> > using the Hermite algorithm. While it works, I'm having some bizarre
>>> >> > issues
>>> >> > with it. Apologies for the cross-post with the PostGIS mailing list.
>>> >> >
>>> >> > In the attached gif of California counties, from left to right, I
>>> >> > have
>>> >> > used
>>> >> > the following tolerance values with the Hermite algorithm:
>>> >> > - original
>>> >> > - 1.0
>>> >> > - 0.08
>>> >> > - 0.01
>>> >> > - 0.00001
>>> >> >
>>> >> > Why do counties disappear entirely as I decrease the tolerance?
>>> >> >
>>> >> This problem has been fixed in GRASS 6.4 only 2 weeks ago (June 13).
>>> >> Please update your GRASS version if possible.
>>> >>
>>> >> Markus M
>>> >>
>>> >>
>>> >> > In the Grass Tools I choose the v.generalize function. I choose
>>> Boundary
>>> >> > as
>>> >> > the feature type (though I've tried checking others, as well as all
>>> >> > of
>>> >> > them
>>> >> > and it doesn't seem to change anything). Everything else is default,
>>> >> > except
>>> >> > for tolerance as notated above.
>>> >> >
>>> >> > When I tested this originally on only Arkansas and Mississippi, I
>>> >> > got
>>> >> > really
>>> >> > nice results. I then tried it on the entire US and had the missing
>>> >> > counties
>>> >> > problem. So I tried only California, and still have the same issue.
>>> >> >
>>> >> > I've tried other algorithms, but this has so far given me the detail
>>> >> > I
>>> >> > want
>>> >> > - of course sans counties! Any thoughts?
>>> >> >
>>> >> > Thanks,
>>> >> > - Chris
>>> >> > _______________________________________________
>>> >> > grass-user mailing list
>>> >> > grass-user at lists.osgeo.org
>>> >> > http://lists.osgeo.org/mailman/listinfo/grass-user
>>> >> >
>>> >> >
>>> >
>>> >
>>>
>