[Ottawa_users] Clipping shapefiles with ogr2ogr

[Christy Nieman]

I have no idea.  My only suggestion would be to try loading the 
shapefile into the GRASS mapset again.

Christy

Alyre Chiasson wrote:
> Hi Christy,
>
> Thanks for the offer of help. I believe I understand your 
> instructions. I have gotten as far as having the GRASS clip mask and 
> the underlying GRASS layer from which I want to retain the underlying 
> feature. So if I could cut out what is under the mask, I would have my 
> desired result. Look good at this stage on the screen. However, when I 
> execute the v.overlay I get the following error, "Database connection 
> not defined for layer 1" No layer is created and QGIS freezes then 
> crashes when I try to exit. Any idea what this message means. Seems 
> like some related to the database pathway? Did something not get 
> properly imported from the shapefiles? How do I correct it?
>
>
> Thanks
>
> Alyre
>
>
>
>
> Loader
>
> Christy Nieman wrote:
>> Hi,
>>
>> I just spent a while working on clipping in QGIS/GRASS, and it is 
>> quite simple.  I used the GRASS Tools plugin for QGIS to load my 
>> shapefiles into a GRASS mapset.  I then converted my mapset's region 
>> to a GRASS vector layer to use as the overlay layer in the clip 
>> (v.in.region - under Vector/Create standard vectors in the QGIS GRASS 
>> tools dialog).  Then I used the v.overlay intersection tool to do the 
>> clip.  This has to be done one layer at a time.
>>
>> If you need more details I can provide a specific example.
>>
>> Christy
>>
>> Alyre Chiasson wrote:
>>> Hello,
>>>
>>> I suspected that something like below was happening, although the 
>>> documentation I read regarding the -spat option presents it as a 
>>> clipping function. I was surprised by the result. I am new to all of 
>>> this and have no computer science background so I am not familar 
>>> with a "bbox" . Although I know Python is a flexible multi platform 
>>> language (I see a lot of references in the Linux world), attempting 
>>> to learn the language to write a script to do this job or to 
>>> understand someone else's script I believe would be beyond my time 
>>> constraints for the current project. I did try to import both shape 
>>> files into GRASS as layers and then tried to overlay (v.overlay) 
>>> them but got error messages which I couldn't understand but were 
>>> basically complaining about missing attributes. Not sure what 
>>> attributes it was referring to. Is there actually no way to do this 
>>> within GRASS or QGIS that retains the feature of the shapefile 
>>> (everthing within the clipping region) without having to learn a 
>>> programming language? Would seem to be a desirable function. Not 
>>> adverse to learning a program language but it would be a much longer 
>>> term goal.
>>>
>>> Thanks
>>>
>>> Alyre
>>>
>>>
>>>
>>> ----- Original Message ----
>>> From: Tyler Mitchell (OSGeo) <tmitchell at osgeo.org>
>>> To: Ottawa (Canada) Local Chapter List <ottawa_users at lists.osgeo.org>
>>> Sent: Monday, February 18, 2008 12:26:36 PM
>>> Subject: Re: [Ottawa_users] Clipping shapefiles with ogr2ogr
>>>
>>> On 18-Feb-08, at 6:22 AM, Paul Spencer wrote:
>>>
>>> > My first guess is that ogr2ogr doesn't clip, it uses the bbox as a 
>>> > spatial selection and keeps anything that intersects with the > 
>>> bbox.  Would this account for your results?
>>>
>>> That's right, it cannot clip, but will select/return all features 
>>> that fit in the area.  You can't use ogr2ogr for clipping but I 
>>> believe you could write a Python script that calls the OGR and GEOS 
>>> functions to do this job.
>>>
>>> Tyler
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