[GRASS-user] Simple example about vector database layers in GRASS

[roy roy]

Thank you Moritz,

i'll try to follow your suggestions

Il 14/06/2016 17:50, Moritz Lennert ha scritto:
> On 14/06/16 09:45, roy roy wrote:
>> I'm reading this wiki page:
>>
>> https://grasswiki.osgeo.org/wiki/Vector_Database_Management#The_concept_of_layers 
>>
>>
>> about the GRASS vector model ad the sketch seems to suggest that I can
>> setup and display
>> several layer given an available or digitized vetor geometry
>>
>> In the example I have a vector object (centroid)
>>
>> id = 16 ( a centroid )
>> (this id is automatically assigned an internal feature id ... can the id
>> be diplayed?
>> how do I know a geometry id?)
>>
>> joined with two distinct attribute tables via category values, i.e.
>>
>> 16 -> 31  (wheat)
>> 16 -> 26  (a name)
>>
>> then there is a boundary with id 1:
>>
>> 1 ->  5 (connected to category 5)
>>
>> what it seems not clear is how you set up such connections:
>>
>> How do i know what id is assigned to a geometry?
>>
>> How do i connect "category" to "id" so that I can assign an owner to
>> each area ?
>
> id's are only used internally. You should only worry about categories. 
> These can either be used as id's (thus you would connect attribute 
> data to features by indicating the category value in the key column of 
> your attribute table), or you can use category values as attributes by 
> themselves (e.g. your example with 31 meaning wheat). The former usage 
> is the classical vector feature <-> attribute table row paradigm. The 
> latter allows to integrate attributes directly into the vector file, 
> thus making some operations faster by completely bypassing the need 
> for attribute database table handling.
>
>
>>
>> what does the following phrase from the page means?
>> "code" value 34 within table "plots" has no corresponding category in
>> layer 2;
>> is the term "category" used instead of "id" (quite confusing ...)
>
> Yes.
>
>>
>> is there a list of modules to perform this operation that could help to
>> better understand?
>
> You use v.category to add / delete / modify category values of 
> features. Category values are also added automatically by most vector 
> import modules.
>
> If you want to add an attribute table using these category features 
> (if no attribute table exists, yet), you can use v.db.addtable. By 
> default this will create an attribute table with only one (key) column 
> 'cat', containing all the category values existing in the chosen layer.
>
> Layers can be seen as information layers. In each information layer 
> you can attribute zero, one or more category values to each feature, 
> and to each information layer you can attach one attribute table.
>
> I hope this makes it a bit clearer.
>
> Moritz
>
>