[GRASS-user] Stranges things after importing a shapefile

[Achim Kisseler]

Hi,

I guess the broken areas in the grass topology have dangles inside.
(zoom a broken area)

use v.clean:
-remove small areas and angles
-delete dangles

Hope it helps,

Achim



Νίκος Αλεξανδρής schrieb:
> Felix Schalck wrote:
>> While still working on my huge map of Europe, I've noticed many
>> differences between the original shapefile and the grass vector layer
>> created after import.
> 
> This (probably) means that grass (i.e. v.clean) did its job and
> corrected topological errors in the shapefile (?).
> 
>> To visualize the problem, I took two
>> Screenshots:
>> -First one is from gqis 1.01, showing the raw
>> shapefile:
>> http://img214.imageshack.us/img214/4884/qgis.png
>>  -Second one from grass 6.4RC5, showing the imported vector
>> layer:
>> http://img4.imageshack.us/img4/8462/grassgis.png
>> Notice how many areas seem broken in the grass layer, and how
>> entire parts of large rivers (take the rhine, for an example) are
>> missing.
> 
> Not so easy to see the differences. Maybe you could (next time)
> highlight them somehow.
> 
>> The initial shapefile was patched together (from a few hundred nasa
>> swbd tiles) using Markus Neteler's script (Thanks again Markus !), and
>> seem to produce good results, at least in Qgis. The import command I
>> used was: v.in.ogr -z -o dsn=/DATA/swbd/shp/tiles/coastlines.shp
>> output=coastlines I somehow had to override the projection, because
>> grass doesn't seem to recognize the projection of the shapefile -
>> although both are of the same WSG84 LAT/LON proj.
>>
>> My question is: what went wrong ?
> 
> Maybe nothing went wrong ;-). Check the original shape(s) there where
> you can locate differences. Are there open polygons for example?
> 
>> Perhaps there is an import step I'm
>> missing, since once the 'clean' command finishes, I get an awful lot
>> of areas without a centroid; I don't know. But it would be nice, for
>> sure, if grass could just translate the shapefile, as it is, and allow
>> me to export a *complete* vector map to inkscape.
> 
> If its only for visualisation purposes, I think you can avoid v.clean.
> Use: v.in.ogr with the flag "-c   Do not clean polygons (not
> recommended)".
> 
> Or maybe v.external?
> 
> Nikos
> 
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