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

[Helena Mitasova]

On Jan 25, 2008, at 2:46 PM, Markus Neteler wrote:

> On Jan 21, 2008 7:59 PM, Helena Mitasova <hmitaso at unity.ncsu.edu>  
> wrote:
>> On Jan 21, 2008, at 11:23 AM, Markus Neteler wrote:
>>> On Jan 21, 2008 4:22 PM, Martin Landa <landa.martin at gmail.com>  
>>> wrote:
>>>> 2008/1/21, Martin Landa <landa.martin at gmail.com>:
>>>>> 2008/1/21, Markus Neteler <neteler at osgeo.org>:
>>>>>> can we convert 3D polygons or tins to a DEM? Possibly a job for
>>>>>> v.to.rast...
>>>>>>
>>>>>> v.to.rast help
>>>>>> ...
>>>>>>           use   Source of raster values:
>>>>>>                 z    - use z coordinate (points or contours only)
>>>>>>
>>>>>> Would it be a big mess to extend that?
>>>>>
>>>>> I guess wouldn't be so complicated to add support also for faces
>>>>> (and
>>>>> 3d lines in general) to v.to.rast... I can try take a look.
>>>>
>>>> just idea,
>>>>
>>>> tin ->
>>>>
>>>> v.to.points -n
>>>> v.clean tool=rmdupl
>>>> v.surf.* -> dem
>>>
>>> well, this would only give an approximation AFAIK.
>>
>> that is right - if you want to preserve the triangle facets in the  
>> TIN
>> you need to do linear interpolation using the 3 points of each
>> triangle facet and you get something like this
>> (this is TIN converted to raster in some old version of Arc)
>>
>> http://skagit.meas.ncsu.edu/~helena/gmslab/interp/F1b.gif
>
>
> I discovered that we have tin.c in vectlib:
>
> int
> Vect_tin_get_z (
>                    struct Map_info *Map,
>                    double tx, double ty,
>                    double *tz, double *angle, double *slope)

that looks promising - I checked v.delaunay and unfortunately it
ignores the z-coordinate even if you give it 3D vector points - so first
the v.delaunay would have to be updated to preserve the z-coordinates
and write 3D vector map rather than 2D (perhaps not too complicated  
task)
and then the simplest way would be just to call the above function  
for each grid point
with Map being the 3D vector area map from v.delaunay.
The computation of slope and aspect is missing  but adding those  
should be easy.

maybe somebody with better C-programming skills could look at it -  
just adding
the z coordinate to v.delaunay would add the missing TIN support to  
GRASS
and the result could be visualized as 3D vector,

Helena


> Markus
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