[GRASS-user] v.patch

GRASS GRASS user
1

Dear list,

I am trying to merge several douzen vectorlayers in a loop. These are representing zones of neighboring municipalities, that have been imported from PostgreSQL and might overlap. Overlapping is not expected to be of relevant size, so it is more important to keep the number of objects as in the PostgreSQL-file.

As far as I understand, there are two possible ways to handle this GRASS:

1.)v.overlay with option=xor/or, this would result in an extended table with attributes a_** and b_**
2.)v.patch which would demand to create unique categories to avoid dissolving zones.

The first solution seems not ideal, as I don't want to enlarge the attribute table within each loop. The content of the layers will change in future, so I also can't hardcode to copy all the content of b_** to a_** with a sql-queries in each loop.
For the second solution I would have to query the cat-number within the loop "echo "SELECT MAX(cat) FROM GR_help1" | db.select". But I don't see how I could pipe this to the value of "v.category option=sum value= XX"

Is there some possibility to do s.th as:
v.category in=gr_help1 out=gr_help1_cat option=sum value="echo 'SELECT MAX(cat) FROM gr_help1' | db.select"

Or is there a simplier solution to this problem, which surely occurs often.

Thanks very much in advance, Patrick

2

On 14/03/12 19:19, Patrick S. wrote:

Is there some possibility to do s.th as:
v.category in=gr_help1 out=gr_help1_cat option=sum value="echo 'SELECT
MAX(cat) FROM gr_help1' | db.select"

If you're on a *nix command line, you can do this:

v.category in=gr_help1 out=gr_help1_cat option=sum value=`echo 'SELECT MAX(cat) FROM gr_help1' | db.select -c`

i.e. use backticks instead of quotes and the -c flag of db.select to make sure that the output is only one value, without column names.

And IIRC you have to use an SQL-backend, i.e. not dbf, for max() to work.

Moritz

3

Thank you Moritz,

that works perfect!

BTW, I saw that you can also write: $( ......) instead of `...`
The result seems to be equivalent.

patrick

On 03/15/2012 10:03 AM, Moritz Lennert wrote:

v.category in=gr_help1 out=gr_help1_cat option=sum value=`echo 'SELECT MAX(cat) FROM gr_help1' | db.select -c`

4

On 15/03/12 11:24, Patrick S. wrote:

Thank you Moritz,

that works perfect!

BTW, I saw that you can also write: $( ......) instead of `...`
The result seems to be equivalent.

Actually $() seems to be the preferred bash way and backticks somewhat deprecated:

http://mywiki.wooledge.org/BashFAQ/082

However, I think that backticks are more standard across shells than $(), but I'll let the real experts respond to that. :wink:

Moritz