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

[Paulo van Breugel]

Keeping both attributes would make sense in many cases of overlapping 
polygons, which, I would guess, is more often about partly overlapping 
polygons due to sloppy digitizing. How is GRASS going to tell what 
feature is more important?

But, in case of duplicate rows in your attribute table, except for the 
cat, you might be able to remove the duplicate rows in the attribute 
table using a simple SQL statement (not sure this works if you use dbf 
as database backend), something along the lines of (google if this 
doesn't work).

DELETE FROM table WHERE cat NOT IN
(SELECT MIN(cat) FROM table GROUP BY XXX);

Whereby XXX would be column with unique values mapping unit (if there is 
not such column, you need to GROUP on a set of columns that together 
uniquely define each mapping unit). You can do this in the db.execute. 
Alternatively, you can use the SELECT statement in the advanced SQL 
query builder in the GRASS Attribute Table Manager to select the 
duplicates and delete them there.

If you use the dbf as database backend and the above doesn't work, you 
can open the dbf file (which you can find in the 'GRASS DB / LOCATION/ 
MAPSET /dbf' folder) in Libreoffice and select all duplicates and 
delete, e.g., using a pivot table. Do not use excel, in my experience 
that may mess up your dbf file.

In all cases, this is just to remove rows that are identical except for 
one column... you'll have to test whether the results make sense in your 
case and you are not messing up your polygon layer.




On 04/02/2012 09:09 PM, David J. Bakeman wrote:
> 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
>>
>
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You can use
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