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

[Rainer M Krug]

On Tue, Jul 8, 2008 at 3:19 PM, Moritz Lennert
<mlennert at club.worldonline.be> wrote:
> 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 ?

I thought the same, but when I do v.overlay, it only takes one value
per polygon over to the new layer. If I could specify the max(YEAR)
query to choose the value, that would be the solution. Just look at
http://grass.osgeo.org/wiki/Vector_Overlapping_Areas - I used the
second solution for the count of the layers per location - but I don;t
have an ide to do it for max(YEAR)


>
> 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
>>>
>>
>>
>>
>
>



-- 
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