[GRASS-user] Determining maximum value of attributes where areas are overlapping in imported vector layer

Moritz Lennert mlennert at club.worldonline.be
Tue Jul 8 09:19:51 EDT 2008


On 08/07/08 15:02, Rainer M Krug wrote:
> On Tue, Jul 8, 2008 at 10:25 AM, Rainer M Krug <r.m.krug at gmail.com> wrote:
>> On Mon, Jul 7, 2008 at 9:37 PM, Markus Neteler <neteler at osgeo.org> wrote:
>>> On Mon, Jul 7, 2008 at 6:28 PM, Rainer M Krug <r.m.krug at gmail.com> wrote:
>>>> On Mon, Jul 7, 2008 at 6:19 PM, Markus Neteler <neteler at osgeo.org> wrote:
>>>>> On Mon, Jul 7, 2008 at 6:11 PM, Rainer M Krug <r.m.krug at gmail.com> wrote:
>>>>>> Hi
>>>>>>
>>>>>> I am back with a problem of imported shape files:
>>>>>>
>>>>>> I have imported a shape file and it contains several areas where
>>>>>> polygons are overlapping. After I managed to obtain the number of
>>>>>> overlapping polygons and exported it to a raster layer
>>>>>> (http://grass.osgeo.org/wiki/Vector_Overlapping_Areas), I would now
>>>>>> need the maximum value of an attribute column for this areas.
>>>>>>
>>>>>> Any ideas how I can achieve this?
>>>>> You can use 'db.select':
>>>>>
>>>>> echo "SELECT max(N1) FROM lidar2" | db.select
>>>>> max(N1)
>>>>> 512834.76000000000931
>>>> Sorry for not being spoecific enough - I want the maximum values as
>>>> ributes in an attribute table.
>>>> As far as I can see, this just gives me the maximum value of the whole
>>>> table - but I need it spatially, i.e. for each area which is
>>>> overlapping, the maximum value.
>>> Then v.to.db might be helpful.
>>> http://grass.osgeo.org/grass64/manuals/html64_user/v.to.db.html
>>>
>>> -> see qcolumn parameter.
>> Thanks - tha is working now.
> 
> I was to fast - it is not working.
> 
> The reason is the following: if a small polygon A with YEAR=2006 is
> completely overlapping with a larger polygon B with YEAR=2000,
> max(YEAR) returns 2006 for the whole of polygon B - which is not what
> I want. I would like to have 2006 for polygon A and 200 for polygon B,
> except of the are where it overlaps with polygon A.
> 
> Any ideas on how to achieve this?

v.overlay + v.to.db ?

Moritz


> 
> Thanks
> 
> Rainer
> 
> 
>> Rainer
>>
>>> Markus
>>>
>>
>>
>> --
>> Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
>> Biology, UCT), Dipl. Phys. (Germany)
>>
>> Plant Conservation Unit
>> Department of Botany
>> University of Cape Town
>> Rondebosch 7701
>> South Africa
>>
> 
> 
> 



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