[GRASS-user] get vector table with unique feature id

[Paulo van Breugel]

On Fri, Apr 3, 2015 at 3:11 PM, Moritz Lennert <mlennert at club.worldonline.be
> wrote:

> On 03/04/15 14:42, Paulo van Breugel wrote:
>
>> I have a shapefile with overlapping polygons. After importing I thus
>> end up with a vector layer with for each unique polygon (feature) one
>> or more category values (i.e., one to many relationship).
>>
>> Now I want to create a second attribute table linked to layer 2 with
>> a cat column with for each polygon (for each internal feature id) an
>>  unique category.
>>
>> Next, I want to use this to count the number of categories linked to
>>  each unique feature id.
>>
>
> If you have overlapping polygons, after running v.in.ogr your map
> already has layer 2 categories which represent for each feature where
> there is overlap the number of overlapping polygons in that space. IIUC,
> this is what you are looking for, or ?
>

Yes, sorry, I was just about to write that this is indeed the case; you
were just ahead of me. In my initial attempt something went wrong, I should
have checked better, but was focused on getting a column with the unique
values per polygon (which as you write below does not seem to work?).


>
>
>> I know this has been asked and answered before, but the solutions
>> (copied below) do not work for me.
>>
>> This worked, I think, in GRASS 6, but it doesn't in G7 because a new
>>  vector map needs to be defined
>>
>>> /v.category op=add layer=2 type=centroid/ /v.to.db type=centroid
>>> option=cat/
>>>
>>
> And what is the problem with defining a new vector map. This new vector
> map will contain all existing layers plus the new layer. But in this
> case it should be:
>
> v.category map op=add layer=3 type=centroid out=map2
>
> You can then create a table for layer 3 with v.db.addtable:
>
> v.db.addtable map2 layer=3 columns="cat2 integer"
>
> and should be able to load the cat values of layer 2 into the table of
> layer 3 with v.to.db,


I knew (ok, I hoped) this should be simple. Thanks


> but this does not seem to work at the moment:
>
> v.to.db map22 layer=3 query_layer=2 op=cat columns=cat2
>
> (there seems to be confusion between map layers, but I don't have time to
> check. Definitely worth a bug report.)
>

I'll create one

>
>
>  The below does not create an attribute table with unique attributes
>> for each unique feature. /> v.in.ogr dsn=test_polygones.shp
>> out=polygons
>> v.db.addtable polygons layer=2
>>
> > v.category polygons option=add
>
>> layer=2 type=centroid out=polygons_tmp
>>>
>> v.to.db polygons_tmp
>>
>>> type=centroid option=cat layer=2 col=cat/
>>>
>>
> For those features where there is overlap, this will not create new
> categories, but upload to the table the existing category which is the
> number of overlapping features.
>

OK, I was confused here, thanks for the explanation

>
>
>
> Moritz
>
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