[GRASS-user] RE: Problem querying layers other than '1' in gis.m

twiens twiens at interbaun.com
Wed Sep 20 11:09:16 EDT 2006

----- Original Message Follows -----
From: Michael Barton <michael.barton at asu.edu>
To: Moritz Lennert <mlennert at club.worldonline.be>, "Patton,
Eric" <epatton at nrcan.gc.ca>
Cc: "'grassuser at grass.itc.it'" <grassuser at grass.itc.it>
Subject: Re: [GRASS-user] RE: Problem querying layers other
than '1' in gis.m
Date: Wed, 20 Sep 2006 07:48:43 -0700

> You CAN link multiple tables to single vector objects.
> That is what layers is all about. You need a separate CAT
> column for each table you link. Each cat column is a
> "layer". It serves as a key to link the object with a
> record in a table.
> I don't know if v.what (used in gui vector querying) will
> query more than layer one. There was some discussion about
> this awhile back and I can't remember the outcome. There
> have been some other new changes to v.what that will
> eventually go into 6.3. If layer support has been added,
> this will go into 6.3 also.

v.what does not support layers.


Your understanding of what layers are supposed to do is
correct, they are multiple linked tables to a single
topology layer, but there are unique cats for each linked

Michael and Eric,

I had been planning on adding support for multiple linked
tables in v.what at some point, but since I deeply dislike
the terminology and think the general approach to this
problem is wrong in the first place I'm not motivated.
Personally I think that attribute management should be left
to the database backend and this so called feature should be
dropped as the terminology is confusing. But.... there is
deep resistance from a number of core developers who
designed and implemented this multiple table link feature,
so that is not likely to happen. The short and long of it
however, is that there are other aspects of GRASS
development which I'm more interested in and I think are
more useful so that is where I have been and will be putting
my limited time. If someone else wants to add "layers" to
v.what that is fine, but I won't be doing it in the
forseable future.

Trevor Wiens

Trevor Wiens
twiens at interbaun.com

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