[GRASS-user] Newbie questions about GRASS data model details

[Anna Petrášová]

On Thu, Mar 17, 2016 at 3:14 PM, Uwe Fischer <gisfisch at t-online.de> wrote:

> Hello all,
>
>
>
> I'm quite new to GRASS (coming from ESRI and MicroStation) and have two
> questions concerning the vector data model. I've read GRASS Wikis, manuals
> and other docs from the web, but I could not find comprehensive answers
> yet. Sorry for the long text ... :-)
>
>
>
> First question: how are the different type determinations for geometry
> objects saved in GRASS? What makes a boundary a boundary and separates it
> from a line? The same for points and centroids.
>
>
>
> I've read about the theoretical differences (eg., boundaries are split at
> intersections and they make up closed rings). But those criteria can apply
> to lines as well.
>
>
>
> Command v.type is able to convert those types of geometry objects. That
> is, lines can be converted to boundaries and vice versa. But how does the
> system determine if an object is a line or a boundary? Where is that
> property saved? Is there an invisible attribute column or flag that
> contains the information? And how and when is that determination changed
> (if not by v.type)?
>
>
>
> Eg. when importing linestrings from DXF to GRASS with import option set to
> "centroid, boundary", the importer will produce lines as well (it will do
> this where linestrings have dangling errors). That means that there must be
> an automated way (running in the background) of determining the geometry
> type. What are the rules it follows?? As you can see, it does not do what
> the user chooses in the import options that were set. If that was the case,
> no lines would come out, just boundaries.
>
>
>
> Second: Up to now, I was not able to understand the concept of layers in
> GRASS. I've read a lot of explanations, most of them telling that a layer
> is a set of category values making up a different view to the underlying
> geometry objects. Using layers, the geometries can be aggregated or
> individually marked in a different, yet parallel manner at the same time.
>
>
>
> But everything described in the manual or Wiki seems to me like one can do
> this using different attribute columns as well. IMHO, that applies even to
> the Wiki example that was just produced to show the advantage of the layer
> concept over attribute columns (example with field borders that are paths
> at the same time).
>
>
>
> So question Nr. 2 is: what is the real advantage of the layer concept over
> a sophisticated set of attribute columns? And where is that layer structure
> saved physically? Can I make it visible? In the end, is it just a set of
> (most of the time invisible) columns??
>

GRASS uses a custom vector format which saves topology as a binary file. I
think v.build allows to dump the topology into a text file if you want to
see it. Also when you switch on vector digitizer, you can see the
difference between lines and boundaries. Attribute tables are not meant to
store geometry or topology.

Anna


>
>
> Thanks and regards, Uwe
>
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