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

[Moritz Lennert]

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 ?

>
> 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, 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.)


> 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.



Moritz