[GRASS-user] v.distance -a : how join output table to output lines?

[Enrico Gallo]

Dear Micha, dear list,


2013/10/3 Micha Silver <micha at arava.co.il>
>
> On 03/10/2013 12:42, Enrico Gallo wrote:
>
> Dear list,
>
> when using (GRASS 6.4.3)  v.distance -a ,  points to lines, you get:
> 1. [output] vector theme, with no attribute table, with N elements
> 2. [table] , with N rows, optionally with source and target cat
>
> Any idea about joining the two outputs without spatial join?
>
> You probably want to look at the "upload=..." parameter


the problem is a bit more complex than using the "upload" parameter, IMHO

when using v.distance looking for all possible connections (not only
the nearest one)

v.distance -a from=vector_from to=vector_to output=connector
upload=cat,dist column=cat_to,distance table=connector_attributes

we get

- a vector output with connecting lines
- an output table with from_cat [by default], cat_to & distance
fields, completely populated

the output table seems to me a perfect attribute table for vector
output, but no common field is given to join the two:

Adding a table (v.db.addtable) and updating the cat field (v.to.db,
cat) of vector output doesn't solve, as I am not able to add an
autoincrement field in output table to perfom a join, without direct
SQL tricks.

Connecting the given output table (v.db.addtable) and then updating a
cat field, doesn't help even, because v.to.db uses an "INSERT"
statement with cat option , not the usual "UPDATE", so you have N
records with no cat + N records with cat but without attributes.

In both cases we are assuming the two "outputs" have the same order
for elements , but I don't know how much robust is this assumption!

But without a robust join, any spatial analysis of connecting lines is
almost impossible...

I am missing something trivial to bypass this impasse?

Regards,
Enrico