[GRASS-user] Unable to get rid of duplicate polygons

[David J. Bakeman]

Markus Metz wrote:
> On Sun, Apr 1, 2012 at 11:45 PM, David J. Bakeman<dbakeman at comcast.net>  wrote:
>    
>> David J. Bakeman wrote:
>>      
>>> Markus Neteler wrote:
>>>        
>>>> On Sun, Apr 1, 2012 at 9:45 PM, David J. Bakeman<dbakeman at comcast.net>
>>>>   wrote:
>>>>          
>>>>> I didn't find an answer in the archives so.
>>>>>
>>>>> I have a shapefile of polygons and some of the polygons are duplicated.
>>>>>   I
>>>>> thought I could use v.clean tool=rmdupl to get rid of these polygons.  I
>>>>> use
>>>>> v.in.ogr to read it in and I get the following:
>>>>>
>>>>> WARNING: 8 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
>>>>>
>>>>> That is correct in that there are 8 duplicate polygons but the only
>>>>> different attribute is the cat which grass added?  What am I missing?  I
>>>>> then tried v.clean tool=bpol,rmdupl and nothing changes it still has the
>>>>> 8
>>>>> duplicates.  What am I doing wrong?
>>>>>            
>>>> I think that you need to add the break tool for v.clean.
>>>>          
>>> Correct that was actually what I was using:  v.clean tool=break,rmdupl
>>>
>>> Looking closer I see that when I run v.clean it doesn't even report the
>>> duplicates that v.in.ogr did but they are still there.  The only thing that
>>> differs in coordinates or attributes is the cat attribute that grass added.
>>>        
>>>>> I am using grass 6.3.0 on fedora core 14 linux.
>>>>>            
>>>> Please note that you can upgrade to grass-6.4.0-4.fc14:
>>>> http://koji.fedoraproject.org/koji/buildinfo?buildID=263115
>>>>          
>>> Thanks I'll see if I can upgrade.
>>>        
>> I upgraded to the 6.4.0 and the results are exactly the same.  The polygons
>> really are identical in every respect except for they have different values
>> in the cat column.  Is there some other grass tool for removing this kind of
>> duplicate?
>>      
> After import with v.in.ogr, there are no duplicate geometries left in
> the vector. What you have now is some areas with two categories
> assigned to them. Removing the duplicates means in this case removing
> one of the two categories, for example with one of the vector
> digitizers.
>    
I'm relatively new to grass but that doesn't make sense.  I started with 
a shapefile with duplicate features.  That is polygons with the exact 
same attributes and geometry (they are identical).  What I thought grass 
could do for me was to read it in and delete one of the duplicates 
without user intervention.  After all it identifies the duplicates so 
why can't it delete one?

Are you saying that the duplicate geometry was deleted but it kept both 
rows even though they were identical as well?  Is there a operation that 
would identify and delete rows that differ in only the cat attribute.
> Markus M
>
>