[GRASS-user] HowTo fix country border topology problem?

[Markus Metz]

Roger André wrote:
> Hi Martin,
>
> Thanks, that is a exactly the problem.  Your page is perfect for
> showing the cause and solution.
>
> One question though, since the solution relies on removing polygons
> whose areas are less than a minimum threshold, how can we do this
> accurately with a global data set?  I suppose I could extract each
> country, reproject it to a local projection, clean out the bad
> polygons, then reproject to lat/lon, and finally patch all of the
> separate countries back into single file.
>   
AFAIK, v.clean tool=rmarea calculates area sizes in square meters for 
latlon locations, no need to reproject, it should be accurate enough.

Markus M

> Roger
> --
>
>
>
> On Sat, Oct 17, 2009 at 12:57 AM, Martin Landa <landa.martin at gmail.com> wrote:
>   
>> Hi,
>>
>> 2009/10/17 Roger André <randre at gmail.com>:
>>     
>>> Tried the v.extract route - it didn't help.  The little segments came
>>>       
>> sure, because there are no 'lines' - just 'boundaries' and 'centroids'
>> which constract 'areas'.
>>
>>     
>>> along.  So I decided to look at the original data and pull one the
>>> polygons from it, to see if I could spot anything strange about it.  I
>>> did find something, maybe, maybe one of the experts here can explain
>>> it.
>>>
>>> Here is what v.clean reports:
>>>
>>> Number of nodes: 55
>>> Number of primitives: 59
>>> Number of points: 0
>>> Number of lines: 0
>>> Number of boundaries: 46
>>> Number of centroids: 13
>>> Number of areas: 13
>>> Number of isles: 9
>>>
>>> This country should have 9 areas, which matches the number of isles.
>>> I'm trying to figure out how to remove the extra areas (rmarea thresh
>>> value is tricky), but I wonder if there is a way to make the areas
>>> match the isles?
>>>       
>> I was facing probably to the similar problem, it's described here [1].
>> Sorry it's in Czech, anyway from the commands you can probably
>> understand what was the problem. I hope it can help you a bit.
>>
>> Martin
>>
>> [1] http://gama.fsv.cvut.cz/wiki/index.php/GRASS_GIS_-_Konzistence_vektorov%C3%BDch_dat
>>
>> --
>> Martin Landa <landa.martin gmail.com> * http://gama.fsv.cvut.cz/~landa
>>
>>     
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