[GRASS-dev] convert 3D polygons or Tins to a DEM?

[Martin Landa]

Hi,

2008/2/4, Hamish <hamish_b at yahoo.com>:
> > Benjamin Ducke:
> > > If we have 3D input points for the TIN construction, then
> > > the resulting triangles should be 3D face primitives not
> > > areas (of which I assumed they were 2D primitives in the
> > > GRASS vector model), shouldn't they?
>
> Martin Landa wrote:
> > AFAIU, face is 3D boundary, kernel is 3D centroid for volume (3D
> > corpus, the topological composition of faces and kernel). So
> > resulting triangles should be zero-height volumes (volumes
> > constracted from one face and kernel). ??
> >
> > > We would then also be talking about kernels, not centroids,
> > > which should be placed in the 3D geometric center of each
> > > traingular face?
> >
> > yes.
Hamish:
> For a long time it has been unclear to me, do kernels live in the
> middle of each face like an area centroid with 3D coordinates, or is it
> for true 3D topology like a single point somewhere within a closed
> volume defined by many faces.

AFAIU, kernel as 3D centroid is a single point within volume (defined
by many faces).

> ie would a cube get a single kernel in the center of it* or six
> kernels, one on each face?

I guess one single kernel in center.

> [*] if kernel-as-center-of-volume that could hold attribute info like
> building address or geological mineral type. For kernel-on-face I can't
> think of what kind of attribute you would want to attach to a wall
> besides something obvious like "north facing".
>
> Thus I'm leaning to the kernel-as-center-of-volume meaning.

I agree with this point.

> In that case a TIN describing a DEM might just get a single kernel, in
> the middle of XY space and placed below the surface to describe the
> interior side. (?)
>
> But it would probably only be meaningful for a closed TIN "sphere" of
> the entire geoid surface.

Or just to ignore kernels, for 3d output support only "Output
tessellation as a 3D graph (faces), not volumes"

?

Martin
-- 
Martin Landa <landa.martin at gmail.com> * http://gama.fsv.cvut.cz/~landa *