[GRASS-dev] v.to.db problems if layer > 1

[Moritz Lennert]

On 27/11/08 09:36, Markus Neteler wrote:
> On Thu, Nov 27, 2008 at 12:10 AM, Moritz Lennert
> <mlennert at club.worldonline.be> wrote:
>> On 26/11/08 11:34, Markus Neteler wrote:
>>> I try to use v.db.addtable with a second layer but v.to.db apparently
>>> fails
>>> (called by v.db.addtable):
>>>
>>> # Spearfish
>>> g.copy vect=roads,myroads --o
>>> v.info -c myroads
>>> v.db.addtable myroads table=extratab layer=2 columns="cat
>>> integer,label varchar(100)"
>>> ...
>>> Updating database...
>>>  100%
>>> 1 categories read from map       <- BAD should be 6
>> Why, roads has no features in layer two, so just by copying it and adding a
>> new table to another layer, you do not add new categories in that layer. You
>> would need 'v.category option=add layer=2' for that.
> 
> Well, I though v.db.addtable would take care of doing the v.category job for
> me (see script).

AFAIKS, it doesn't. I'm not sure I think that this would be a good idea, 
either, as it would indiscriminately attribute categories to all 
features in the map (e.g. boundaries and centroids, not only centroids), 
unless you add a type option.

I have the feeling that this would only complicate issues as too much 
would happen without the user really knowing, so I have a preference for 
leaving it to the responsibility of the user to create category values.

Probably the docs could be better about all this, though.

> 
>>> ...
>>> v.db.select myroads layer=2
>>> cat|label
>>>                        <- empty, should have 6 cats
>>>
>>> # compare layer1:
>>> v.db.select myroads layer=1
>>> cat|label
>>> 0|no data
>>> 1|interstate
>>> 2|primary highway, hard surface
>>> 3|secondary highway, hard surface
>>> 4|light-duty road, improved surface
>>> 5|unimproved road
>>>
>>> -----------
>>> # calling directly v.to.db
>>> v.to.db myroads layer=2 option=cat col=cat qlayer=1
>> Shouldn't this be:
>>
>> v.to.db myroads layer=2 option=query col=cat qlayer=1 qcolumn=cat

And to respond to Peter's original query, i.e:

> 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? 


it should actually be (assuming that the varchar attribute in layer 1 is 
called 'name':

v.db.addcol FOO layer=2 col='name varchar(Size)'
v.to.db myroads layer=2 option=query col=name qlayer=1 qcolumn=name

Moritz