----- Original Message Follows -----
From: Michael Barton <michael.barton@asu.edu>
To: Moritz Lennert <mlennert@club.worldonline.be>
Cc: Trevor Wiens <twiens@interbaun.com>, GRASS5
<grass5@grass.itc.it>, Radim Blazek
<radim.blazek@gmail.com>, Helena Mitasova
<hmitaso@unity.ncsu.edu>, Martin Landa
<landa.martin@gmail.com>
Subject: Re: [GRASS5] Layers Clarification
Date: Tue, 07 Mar 2006 10:43:28 -0700
>> I want to embellish on Radim's response a bit.
>>
>> ONE vector object can join with can be joined with ONE
attribute table for >> EACH "layer" it has. If it has ONE
layer, it can join with ONE table, if it >> has TWO layers
, it can join with TWO tables. >>
Michael,
In Radim's example of v.clean
1
+-----------+
2
+----------------+
v.clean yielding
1 1,2 2
+------+----+------------+
According to Radim's text the result is still in one layer.
Thus I must conclude that although GIS objects can only
connect to one table per layer, GIS objects can connect to
more than one row in the linking table thus supporting many
to one joins as well as one to one or one to many.
This seems odd. What am I missing here.
T
On 3/7/06, twiens <twiens@interbaun.com> wrote:
> >> I want to embellish on Radim's response a bit.
> >>
> >> ONE vector object can join with can be joined with ONE
> attribute table for >> EACH "layer" it has. If it has ONE
> layer, it can join with ONE table, if it >> has TWO layers
> , it can join with TWO tables. >>
Michael,
In Radim's example of v.clean
1
+-----------+
2
+----------------+
v.clean yielding
1 1,2 2
+------+----+------------+
According to Radim's text the result is still in one layer.
Thus I must conclude that although GIS objects can only
connect to one table per layer, GIS objects can connect to
more than one row in the linking table thus supporting many
to one joins as well as one to one or one to many.
Yes. One geometry object can be linked to more rows in attribute table.
In that case the geometry object has more categories (the same layer)
each pointing to one row in the attribute table. One category can only
link to one row, no more. The relation geometry-table can be
one-to-many. The relation category-table can only be
on-to-one or many-to-one.
It is probably much easier get idea from a picture:
http://mpa.itc.it/radim/g51/bridge.png
It covers all possible cases I think.
Radim
This seems odd. What am I missing here.
T
On Wed, 8 Mar 2006 09:50:28 +0100
"Radim Blazek" <radim.blazek@gmail.com> wrote:
Yes. One geometry object can be linked to more rows in attribute table.
In that case the geometry object has more categories (the same layer)
each pointing to one row in the attribute table. One category can only
link to one row, no more. The relation geometry-table can be
one-to-many. The relation category-table can only be
on-to-one or many-to-one.
It is probably much easier get idea from a picture:
http://mpa.itc.it/radim/g51/bridge.png
It covers all possible cases I think.
Radim and others,
Thank you for taking the time to explain this. Is it possible to
manually add more than one category to a geometry object or is this
only done through automated means such as v.clean?
Thanks. Once I get this all straight in my head I will assemble this
into a single document. Do you think my idea of having a statement on
the vector module index page with a link to a more detailed explanation
is a good idea or would somewhere else be more appropriate?
T
--
Trevor Wiens
twiens@interbaun.com
The significant problems that we face cannot be solved at the same
level of thinking we were at when we created them.
(Albert Einstein)
On 3/8/06, Trevor Wiens <twiens@interbaun.com> wrote:
> Yes. One geometry object can be linked to more rows in attribute table.
> In that case the geometry object has more categories (the same layer)
> each pointing to one row in the attribute table. One category can only
> link to one row, no more. The relation geometry-table can be
> one-to-many. The relation category-table can only be
> on-to-one or many-to-one.
>
> It is probably much easier get idea from a picture:
> http://mpa.itc.it/radim/g51/bridge.png
> It covers all possible cases I think.
>
Radim and others,
Thank you for taking the time to explain this. Is it possible to
manually add more than one category to a geometry object or is this
only done through automated means such as v.clean?
Yes, it is possible,
v.digit: Display categories -> Add new
QGIS: Edit Attributes -> New
Thanks. Once I get this all straight in my head I will assemble this
into a single document. Do you think my idea of having a statement on
the vector module index page with a link to a more detailed explanation
is a good idea or would somewhere else be more appropriate?
I think that it could be incorporated into
http://grass.itc.it/grass61/manuals/html61_user/vectorintro.html
(it is currently linked from main index) but you can create
a new document as well.
Radim
T
--
Trevor Wiens
twiens@interbaun.com
The significant problems that we face cannot be solved at the same
level of thinking we were at when we created them.
(Albert Einstein)
Radim Blazek wrote:
On 3/8/06, Trevor Wiens <twiens@interbaun.com> wrote:
Yes. One geometry object can be linked to more rows in attribute table.
In that case the geometry object has more categories (the same layer)
each pointing to one row in the attribute table. One category can only
link to one row, no more. The relation geometry-table can be
one-to-many. The relation category-table can only be
on-to-one or many-to-one.
It is probably much easier get idea from a picture:
http://mpa.itc.it/radim/g51/bridge.png
It covers all possible cases I think.
Radim and others,
Thank you for taking the time to explain this. Is it possible to
manually add more than one category to a geometry object or is this
only done through automated means such as v.clean?
Yes, it is possible,
v.digit: Display categories -> Add new
QGIS: Edit Attributes -> New
Thanks. Once I get this all straight in my head I will assemble this
into a single document. Do you think my idea of having a statement on
the vector module index page with a link to a more detailed explanation
is a good idea or would somewhere else be more appropriate?
I think that it could be incorporated into
http://grass.itc.it/grass61/manuals/html61_user/vectorintro.html
(it is currently linked from main index) but you can create
a new document as well.
Trevor,
I think that this would be a good place where to include the layer info.
And you could also explain what the level1 and level2 means, as it comes
in error messages and v.in.ascii and v.surf.rst now allow to work with just level1 data. Then the sentence about v.build can be modified saying that v.build can also be used to build topology (level2) vector map, if
it is missing.
Thanks to all for calrifying the layers concept,
Helena
Radim
T
--
Trevor Wiens
twiens@interbaun.com
The significant problems that we face cannot be solved at the same
level of thinking we were at when we created them.
(Albert Einstein)
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