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

[Benjamin Ducke]

I need some clarifications before I can get to work on this,
because I am getting a bit confused.

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?
We would then also be talking about kernels, not centroids,
which should be placed in the 3D geometric center of each
traingular face?
Or am I getting this totally wrong?

Also, it seems to me that for a 3D delaunay triangulation
the algorithm has to search for the nearest point in 3D
space, because if it uses a 2D search, than it will be fooled
by undercutting points and triangulate the wrong
point triplets (I admit that undercuts don't usually appear in terrain
surface modelling, but I have some data where they do).

I guess the problem of building topology for massive data sets should
be solved in the GRASS vector libraries, not in individual modules?

Benjamin

Helena Mitasova wrote:
> In relation to handling of large data sets - changing the algorithm
> may not solve the issue. I haven't tried it with larger data set but
> based on my previous experiences it may get stuck on topology building.
> So that would need to be solved first (and you cannot skip topology
> this time, as we did with points, because the result are areas and
> nothing would work with the resulting map without topology).
> 
> As for 3D - Benjamin, if you could just add the original z value to the
> output vertices (the value for centroid can be computed by calling the
> tin.c ) that will be sufficient to get the basic functionality and display
> the resulting TIN in 3D. For more sophisticated triangulations using
> the existing libraries would be a more comprehensive solution,
> as suggested by Markus (as long as we can make them part of GRASS
> distribution)
> 
> Helena
> 
> On Jan 28, 2008, at 4:40 AM, Benjamin Ducke wrote:
> 
>> I noticed this one too a few days back when I was looking for a
>> 3D hull algorithm. Looks like a promising collection of algorithm
>> for GRASS.
>>
>> However, for a quick fix of v.delaunay, as suggested by Helena:
>> Might it not be enough to replace the 2D distance measure in
>> v.delaunay with a 3D distance, so it will do the tesselation in
>> 3D space correctly?
>> I could try and find some time to look into that if you think
>> it feasible.
>>
>> Benjamin
>>
>>
>>
>> Markus Neteler wrote:
>>> On Jan 28, 2008 9:29 AM, Maris Nartiss <maris.gis at gmail.com> wrote:
>>>> Fixing large dataset and 3D support problems would be a nice
>>>> improvement of GRASS vector support :)
>>>
>>> Possibly we need to substitute the algorithm? I found for example
>>> http://www.qhull.org/
>>> "Qhull computes the convex hull, Delaunay triangulation, Voronoi
>>> diagram, halfspace intersection about a point, furthest-site Delaunay
>>> triangulation, and furthest-site Voronoi diagram. The software runs in
>>> 2-d, 3-d, 4-d, and higher dimensions. Qhull implements the Quickhull
>>> algorithm for computing the convex hull. It handles roundoff errors
>>> from floating point arithmetic. Qhull also computes volumes, surface
>>> areas, and approximations to the convex hull."
>>>
>>> (seems to be GPL compliant)
>>>
>>> Markus
>>> _______________________________________________
>>> grass-dev mailing list
>>> grass-dev at lists.osgeo.org
>>> http://lists.osgeo.org/mailman/listinfo/grass-dev
>>>
>>>
>>
>> -- 
>> Benjamin Ducke, M.A.
>> Archäoinformatik
>> (Archaeoinformation Science)
>> Institut für Ur- und Frühgeschichte
>> (Inst. of Prehistoric and Historic Archaeology)
>> Christian-Albrechts-Universität zu Kiel
>> Johanna-Mestorf-Straße 2-6
>> D 24098 Kiel
>> Germany
>>
>> Tel.: ++49 (0)431 880-3378 / -3379
>> Fax : ++49 (0)431 880-7300
>> www.uni-kiel.de/ufg
>>
>> _______________________________________________
>> grass-dev mailing list
>> grass-dev at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/grass-dev
> 
> 
> 

-- 
Benjamin Ducke, M.A.
Archäoinformatik
(Archaeoinformation Science)
Institut für Ur- und Frühgeschichte
(Inst. of Prehistoric and Historic Archaeology)
Christian-Albrechts-Universität zu Kiel
Johanna-Mestorf-Straße 2-6
D 24098 Kiel
Germany

Tel.: ++49 (0)431 880-3378 / -3379
Fax : ++49 (0)431 880-7300
www.uni-kiel.de/ufg