[GRASS-user] Questions: Clipping polygon layer to the outer boundary of another layer...

[Rick Reeves]

Thanks Hamish - This Worked Great.

Hey, while I am writing, can you recommend any GRASS methods for 
cleaning line feature
topology, besides v.clean? I am trying to repair a lot of missed 
connections in the USGS
NHP2 stream network data so that I can accurately compute stream 
distances with v.net.path.

At this point I may be forced to resort to Arc GIS Network Analyst to 
diagnose the problems,
but would MUCH rather use the GRASS tools.

A final question: Is anyone aware of a version of the NHP stream dataset 
that HAS been
cleaned and repaired to support distance calculations? This would not be 
a trivial task,
even for portions of the dataset.

Thanks,
Rick R
Hamish wrote:
> Rick Reeves wrote:
>   
>>>> I'm using v.in.ogr to clip a polygon set (in this case, county 
>>>> boundaries) to the boundary of a (rectangular) raster image.
>>>>
>>>> As far as I know, this is the only way to do this; I tried to
>>>> create a new location with the same boundaries and import the
>>>> polygon shapefile into the location, but clipping did not occur
>>>> as expected.
>>>>
>>>> In any case, v.in.ogr did not clip the 'boundary polygons' to the
>>>> the edge of the raster image, but included the entire polygon.
>>>> Only excluded polygons completely outside of the bounding box
>>>> are clipped.
>>>>         
>
> Hamish:
>   
>>> I assume you used 'v.in.ogr spatial='. This works like v.select.
>>>       
> R:
>   
>>>> There MUST be a way to clip all polygons to the bounding box edge.
>>>> Can any one suggest a way?
>>>>         
> H:
>   
>>> v.in.region + v.overlay.
>>>       
>
> Moritz Lennert wrote:
>   
>> Or v.in.region + v.select if you don't wan't to merge attribute
>> tables.
>>     
>
> v.select will act the same way as 'v.in.ogr spatial=' (or a personal
> favourite: 'v.in.ogr -r') which was what the original question was
> hoping to avoid. ie v.select leaves features in or out, while v.overlay
> both allows or omits features AND modifies any features which cross.
>
>
> Hamish
>
>
>
>
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