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

[Moritz Lennert]

On 08/07/08 18:14, Rainer M Krug wrote:
> 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)

Well, can't you then do a second overlay with the result of the first + 
the third layer of polygons ?

Moritz