[GRASS-user] How to copy vector columns between databases

[Moritz Lennert]

On 25/11/08 16:29, peter.loewe at gmx.de wrote:
> Hi,
> 
> I have a vector layer FOO which is linked to two tables in layers 1 and 2.
> The categories for each vector element are different in layer 1 and (e.g. a certain area may have the cat value "51" in layer 1 and a cat value of "42" in layer 2).
> Let's assume that layer one has a VARCHAR column containing the names of the region (e.g. database_layer_1: 51,"Wolfenstein" database_layer_2: 42 )
> 
> If a new VARCHAR column is added to layer 2 by v.db.adcol,
> how can the the names from layer 1 be copied into it?
> 
> [Goal: database_layer_1: 51,"Wolfenstein" database_layer_2: 42,"Wolfenstein" ]
> 
> Unfortunately, v.db.update seems only to work within one layer.
> 
> An UPDATE/SELECT SQL-statement will not be possible unless a table can be produced which holds the categories for both database layers for each geometry element.
> 
> How can this be solved ?

At this stage, the only way I can see to solve this problem is using 
v.build option=cdump. This gives you something like this:

---------- CATEGORY INDEX DUMP: Number of layers: 2 
---------------------------- ----------
Layer      1  number of unique cats:       6  number of cats:       6 
number of  types: 1
-------------------------------------------------------------------------------- 
----------
             type |     count
                1 |         6
  category | type | line/area
         1 |    1 |         1
         2 |    1 |         2
         3 |    1 |         3
         4 |    1 |         4
         5 |    1 |         5
         6 |    1 |         6
-------------------------------------------------------------------------------- 
----------
Layer      2  number of unique cats:       6  number of cats:       6 
number of  types: 1
-------------------------------------------------------------------------------- 
----------
             type |     count
                1 |         6
  category | type | line/area
         1 |    1 |         1
         3 |    1 |         2
         5 |    1 |         3
         7 |    1 |         4
         9 |    1 |         5
        11 |    1 |         6
--------------------------------

So, you see that the line/area id can be used to find the correspondance 
between the category values in both layers (i.e. in this case cat 3 in 
layer 1 is the same object as cat 5 in layer 2.

Unfortunately, there is no script friendly output from v.build 
(ToDo...), but still it should be possible to wrap this into a script 
with some python or awk magic. The easiest would probably be to use a 
real SQL backend (i.e. not dbf) and to add a column with the line id to 
each feature (i.e. 'v.db.update col=lineid value=$line where="cat=$cat"' 
where $line and $cat are the variables of the script read from the 
v.build ouput...

If you come up with a script, I think that this would be very useful. 
One thing I have been confronted with before was the desire to create a 
second layer with exactly the same category values (which didn't start 
at 1 and which had no regular step as they were statistical id codes of 
municipalities) which currently isn't possible.

But adding a clayer option to v.db.update might be another solution.

Moritz