[GRASS-user] confused by the GRASS Programmer's topology examples...

Markus Metz markus.metz.giswork at gmail.com
Fri Apr 12 01:27:58 PDT 2013

On Thu, Apr 11, 2013 at 11:32 AM, G. Allegri <giohappy at gmail.com> wrote:
> Hi all,
> I'm reading the GRASS7 programmer's manual, to update my experience with
> GRASS (I'm going to use it for some network analysis, and probably I will
> need to extend it)

You would need to read the  programmer's manual only if you want to
develop GRASS. The user manual should explain everything needed to use

> Looking at the topology examples [1] I fill a bit confused by the "types" of
> nodes and lines, which vary between the different kind of topological
> structures.
>  - Point example: a point doesn't have nodes but have a line (P_LINE), with
> type GV_POINT.
> It's enough for me to get confused :)

A point (GV_POINT) is not attached to nodes (in GRASS 7). P_line is an
internal structure that holds information about points, lines,
boundaries, centroids, faces. Or in your words, a structure P_line
does not hold information about nodes if it is of type GV_POINT.

> A P_LINE structure has a void * topo opaque element [2], which will point to
> the relative topology structure depending on the type. In case of a G_POINT
> type, what is the topology structure attached to the topo pointer?
>  - Line example: two nodes with type GV_LINE. A P_NODE doesn't have a type
> [3], so how can it be... a GV_LINE? :(

It seems you are mixing internal structures (P_line, P_node) and
In the example, there is one line (GV_LINE), and the end points of the
line are attached to nodes. Two end points -> two nodes, if the end
points are different. The "type = 2 (GV_LINE)" in the example for the
node refers to the feature type attached to the node.

Markus M

> I'm missing something. Could you help to understand it?
> Thanks a lot,
> giovanni
> [1] http://grass.osgeo.org/programming7/vlibTopology.html#vlibTopoExamples
> [2] http://grass.osgeo.org/programming7/structP__line.html
> [3] http://grass.osgeo.org/programming7/structP__node.html
> --
> Giovanni Allegri
> http://about.me/giovanniallegri
> blog: http://blog.spaziogis.it
> GEO+ geomatica in Italia http://bit.ly/GEOplus
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