On 25/11/08 16:29, [EMAIL PROTECTED] 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
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