[GRASS-user] v.db.connect additional attribut table

[Moritz Lennert]

On Sat, July 4, 2009 17:38, Falko Engel wrote:
> Dear List,
>
> I am still running into problems with connecting an additional attribute
> table to an existing map.
> I am using postgres to store all tables in. This is my setup: GRASS 6.4
> RC5, Suse 11.1, and Postgresql 8.3.7.
>
> I have a polygon (area, centroid) map containing my whole study area.
> There are about 10000 polygones in this map.
> In an additional table with about 300 rows I have extra information for
> some of the posygones of the original geometry.
>
> In the map's original attribute table there is a colum with an
> identifier that is also used in the additional table. It is in string
> format.
>
> Can someone tell me how I can connect the table to the map as a second
> layer?
>
>
>
> Heres what I already did:
>
> # Reclass cats of map according to identifier column
> v.reclass input=map output=map_reclass column=identifier
>
>
>
> # Using SQL
> ------------------------------------------------------------------
> # Update an integer column "table_id" in the new table with the cat
> values of the map according to the identifier column
>
> UPDATE new_table
> SET table_id = map_reclass.cat
> FROM map_reclass
> WHERE new_table.identifier = map_reclass.identifier
> #
> -------------------------------------------------------------------------------------------
>
> v.db.connect -o map=map_reclass key=table_id layer=2 table=new_table
> v.category map_reclass option=add layer=2 output=map_reclass_layer2

By default, v.category option=add will create new categories starting from
1 and incremented by 1. These category numbers are probably very different
from the contents of your table_id column, and, therefore, your connection
will show no attributes as there are no lines in the table which have a
table_id equal to a category number in your map.

To use the same category values as in another layer, you can insert the
cat values from the database table in one layer to a column in your new
layer using 'v.to.db ... option=query qlayer=OtherLayer
qcolumns=IdColumnInOtherLayer'. If you want you can then reclass according
to that column.

Moritz