Yes, thanks, makes sense Dylan. I'll need some time to experiment before I
report back on success/failure.
Cheers,
~ Eric.
-----Original Message-----
From: Dylan Beaudette
To: grassuser@grass.itc.it
Cc: Patton, Eric; 'Moritz Lennert '
Sent: 9/20/2006 1:35 PM
Subject: Re: [GRASS-user] RE: Problem querying layers other than '1' in gi
s.m
On Wednesday 20 September 2006 06:44, Patton, Eric wrote:
Moritz:
>You do not have to duplicate the points, but IIUC you have to assign
a
>category value for each point in each layer. (See
http://mpa.itc.it/markus/grass63progman/Vector_Library.html#vlib_catego
r
>ies_layers)
I'm probably misunderstanding the use of layers in general, but I
thought
it was analogous to 'joined tables' in Arc (one geometry, many
tables).
Thinking about how I linked the tables to the vector, I just realized
how I
may have made a mistake. The vector in question is a series of points
representing sample locations. Layer one contains a table with cat,
Timestamp, Easting and Northing. I wanted to create a second layer
linking
to a table containing biological analyses of the sample data. The only
column in common between layer one and layer two is the timestamp
field. Do
I also need a field in layer 2 telling what cat in layer 1 it belongs
to?
Maybe that is why I can't query anything from layer 2. If I am right
here,
how can I construct an SQL statement to populate layer 2 with the
corresponding cat from layer 1 where the timestamps in each table
match?
>BTW: have you tried to query on the command line (using d.mon x0 to
open
>a monitor) ?
>
>MoritzYes, I tried the same thing using d.what.vect -e and only layer one's
attributes are queried.~ Eric.
Eric, the way that i usually approach this type of problem in GRASS is
as
follows:
1. store my attribute data in a RDBMS such as mysql, postgresql, sqlite,
etc.
2. create some vector data with at least two columns: 'cat' and 'id'
'cat' is used by v.db.connect to associate a row with a vector feature
'id' is used to associate other attribute data with a row
3. create a new, composite table (from two existing tables 'vector'
and 'data') with something like:
create table new_table as
select vector.cat, vector.id, data.time, data.temp
from vector, data
where vector.id = data.id ;
4. then hookup new_table to the original vector file with v.db.connect
make sense?
--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341