[GRASS-dev] convert 3D polygons or Tins to a DEM?
hamish_b at yahoo.com
Mon Feb 4 01:39:39 EST 2008
> 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?
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.
ie would a cube get a single kernel in the center of it* or six
kernels, one on each face?
[*] 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.
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.
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