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

Wed Jul 9 06:45:56 EDT 2008

On Wed, Jul 9, 2008 at 12:30 PM, G. Allegri <giohappy at gmail.com> wrote:
> It's not clear to me your situation...
> You have a singol vector layer with overlapping polygons? Did you
> import it as Level 1 (non-topological)?
>

I imported it with

v.in.ogr dsn=inputDir layer=fireHistory output=fireHistory --overwrite

and I get at the end the following warning:
.
.
.
WARNING: 703 areas represent more (overlapping) features, because polygons
         overlap in input layer(s). Such areas are linked to more than 1
         row in attribute table. The number of features for those areas is
         stored as category in layer 2
225 input polygons
Total area: 2.033213e+08 (1220 areas)
Overlapping area: 6.822353e+07 (703 areas)
Area without category: 1.267486e+06 (65 areas)

What I want is to

a) identify these areas which have overlapping features (done via
layer 2) and identify how many features overlap (also done, see
http://grass.osgeo.org/wiki/Vector_Overlapping_Areas)

b) Get the maximum value of the field YEAR in the attribute table for
each feature


> 2008/7/9 Rainer M Krug <r.m.krug at gmail.com>:
>> On Tue, Jul 8, 2008 at 6:34 PM, Moritz Lennert
>> <mlennert at club.worldonline.be> wrote:
>>> 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 ?
>>
>> There seems to be still a misunderstanding, as all the polygons are in
>> the same grass vector layer and if I overlay the first overlay again
>> with the original layer, I the value of b_YEAR just becomes repeated.
>>
>> I will see that I construct a simple example and post it here.
>>
>> Thanks
>>
>> 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 AfricaWARNING: 703 areas represent more (overlapping) features, because polygons
         overlap in input layer(s). Such areas are linked to more than 1
         row in attribute table. The number of features for those areas is
         stored as category in layer 2
225 input polygons
Total area: 2.033213e+08 (1220 areas)
Overlapping area: 6.822353e+07 (703 areas)
Area without category: 1.267486e+06 (65 areas)

>> _______________________________________________
>> grass-user mailing list
>> grass-user at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/grass-user
>>
>


-- 
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