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

[Roger André]

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.

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
>