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

GRASS GRASS user
1

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.

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

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

2

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

3

On Fri, Apr 3, 2015 at 3:11 PM, Moritz Lennert <mlennert@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

4

On Fri, Apr 3, 2015 at 3:22 PM, Paulo van Breugel <p.vanbreugel@gmail.com>
wrote:

On Fri, Apr 3, 2015 at 3:11 PM, Moritz Lennert <
mlennert@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

Created a ticket: Ticket #2641

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