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

[Rainer M Krug]

On Tue, Jul 8, 2008 at 4:21 PM, Moritz Lennert
<mlennert at club.worldonline.be> wrote:
> On 08/07/08 15:33, Rainer M Krug wrote:
>>
>> 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)
>
> Not sure I understand. v.overlay operator=or should create new entities
> which are (to use your example): a piece of polygon A where there is only
> polygon A, a piece of polygon B where there is only polygon B and the common
> area between the two. In the table, you should then have the columns of both
> (a_cat, b_cat and a_YEAR, b_YEAR, etc) some of which will be null where you
> only have one of the two polygons. You can then query this table with
> something like this:
>
> select a_YEAR ... where b_YEAR is null  or a_YEAR<b_YEAR

Yes, that is right. It seems to be working for two overlapping
polygons, but as soom as I have three overlapping polygons, I only get
two YEARS back (a_YEAR and b_YEAR - as the overlay is done between two
maps)

Rainer


>
> Moritz
>



-- 
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