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

GRASS GRASS user
,
1

Roger,

If you load the data into PostGIS, this sort of operation is relatively
trivial....

One of the reasons I don't use GRASS is that projections are tied to
workspaces, instead of datasets & I use data in several projections
concurrently, I don't want to have to keep several copies of the same
dataset as required by GRASS.

If you are interested in looking at this approach, feel free to contact
me.

Brent Wood

Cheers,

  Brent Wood

Brent Wood
DBA/GIS consultant
NIWA, Wellington
New Zealand

Roger André<randre@gmail.com> 10/18/09 6:03 AM >>>

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@gmail.com>
wrote:

Hi,

2009/10/17 Roger André <randre@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_vektorových_dat

--
Martin Landa <landa.martin gmail.com> * http://gama.fsv.cvut.cz/~landa

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2

Hi,

2009/10/18 Brent Wood <b.wood@niwa.co.nz>:

One of the reasons I don't use GRASS is that projections are tied to
workspaces, instead of datasets & I use data in several projections
concurrently, I don't want to have to keep several copies of the same
dataset as required by GRASS.

right, projection is related to GRASS location. Or better to say all
data layers from one GRASS location need to be in the same projection.

If you have data layers in different projections, for every projection
you need to have separate GRASS location and re-project vector layers
by manually (v.proj). This could be simplified with GRASS 7 and better
OGR support. Then you could access different e.g. PostGIS layers
directly. Those layers would be reprojected by PostGIS into projection
matching GRASS location settings.

Martin

--
Martin Landa <landa.martin gmail.com> * http://gama.fsv.cvut.cz/~landa