[GRASS-user] Seeking vector layer tutorial

[Ken Mankoff]

Hi Markus,

Thank you for your reply and help. I'm starting to gain a better understanding of the power and complexity of the GRASS vector implementation.

  -k.

On 2019-03-10 at 21:09 +0100, Markus Metz
<markus.metz.giswork at gmail.com> wrote...
> On Fri, Mar 8, 2019 at 9:22 PM Ken Mankoff <mankoff at gmail.com> wrote:
>>
>> Hello,
>>
>> I'm trying to gain proficiency w/ GRASS vectors & layers. I've read
> https://grass.osgeo.org/grass76/manuals/vectorintro.html and
> https://grasswiki.osgeo.org/wiki/Vector_Database_Management a few
> times, but am still not clear on how to create layers.
>>
>> I have three vector layers: BASINS, STREAMS, and OUTLETS. All basins
>> have
> outlets. All streams have outlets. Not all basins have streams (some
> small basins have no streams but still have an outlet).
>>
>> I can merge these three with v.patch, but they're all in one layer. I
> thought it might make sense for each to exist on its own layer. Or
> perhaps my misconception is that they cannot - it is the attribute
> tables that exist on separate layers?
>>
>> If someone can provide an example how to combine these there vectors,
>> or
> link to a tutorial, I would be grateful.
>
> In this example, there are three vectors, each with one layer which is
> probably for all layer number 1.
>
> If you want to patch these three vectors together such that each input
> vector with one layer ends up in one individual layer in the output
> vector, you need to do some preparation, e.g.:
>
> Keep vector layer 1 for BASINS.
>
> Change vector layer 1 for STREAMS to layer 2 with v.category
> in=STREAMS out=STREAMS_l2 op=chlayer layer=1,2
>
> Change vector layer 1 for OUTLETS to layer 3 with v.category
> in=OUTLETS out=OUTLETS_l3 op=chlayer layer=1,3
>
> patch them together with v.patch in=BASINS,STREAMS_l2,OUTLETS_l3
> out=...
>
> In order to copy attributes, you need to copy the attribute tables of
> the input vectors BASINS, STREAMS, and OUTLETS.
>
> The attribute tables linked to the original input vectors are printed
> with v.db.connect -p or v.db.connect -g.
>
> Now copy the tables with db.copy and link them to corresponding layer
> of the patched output using v.db.connect.
>
> HTH,
>
> Markus M