[postgis-users] Recommendations with GRASS v.generalize() on Tiger Data

[Dan Putler]

Hi Chris,

Your question on QGIS and GRASS was posted to the PostGIS user list. 
While you might get a response, this seems like a question for the QGIS 
user list since it actually doesn't involve PostGIS.

Dan

On 06/29/2011 03:55 PM, Christian Guirreri wrote:
> Narrowed down some test to using the "Hermite" algorithm, but I'm 
> having some bizarre issues with it.
>
> In the attached gif of California counties from Tiger data, from left 
> to right, I have the following tolerance values:
>  - original
>  - 1.0
>  - 0.08
>  - 0.01
>  - 0.00001
>
> Why do counties disappear entirely as I decrease the tolerance?
>
> Setup is QuantumGIS with GRASS. In the Grass Tools I choose the 
> v.generalize function. I choose Boundary as the feature type (though 
> I've tried checking others and it doesn't seem to change anything). 
> Everything else is default, except for tolerance as notated above.
>
> When I tested this original 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 what I want. 
> Any thoughts?
>  - Chris
>
>
> On Wed, Jun 29, 2011 at 5:10 PM, Christian Guirreri 
> <christian at guirreri.com <mailto:christian at guirreri.com>> wrote:
>
>     I'm currently going through the GRASS v.generalize() function's
>     various parameters in QuantumGIS. There's so many options, I'm not
>     entirely sure what's best. Has anyone tried this on Tiger 2010
>     counties or district data? Any particular recommendations?
>
>     What's most important to me, as mentioned in a previous thread, is
>     that there are no gaps between counties/districts in a similar
>     fashion to MapShaper.
>
>     Thanks,
>      - Chris
>
>