[GRASS-user] Why does v.split convert 3D vector to 2D?

[Markus Neteler]

On Sun, Mar 7, 2010 at 8:50 PM, Dwight Needels <needels at translucida.com> wrote:
> On Mar 7, 2010, at 2:10 PM, Markus Neteler wrote:
>> On Sun, Mar 7, 2010 at 7:13 PM, Dwight Needels <needels at translucida.com>
>> wrote:
>>> I have a 3D vector [test_3D] created using v.drape, confirmed as 3D with
>>> v.info. I used v.split to split the polyline at the vertices:
>>>
>>> v.split input=test_3D output=test_3D_split vertices=2
>>>
>>> The resulting vector [test_3D_split] is a 2D vector (i.e. it lost the z
>>> geometry data according to v.info, and v.to.db reports all slopes = 0).
...
> Markus, I would appreciate if you would be able to compile and test for me.
> Here is a Spearfish example that shows the same behavior (using v.info to
> test 2D/3D vector status).
>
> v.drape input=t_hydro rast=elevation.dem output=t_hydro_3D method=cubic
> v.split input=t_hydro_3D output=t_hydro_split vertices=2

Looks good:

v.info t_hydro_split | grep 3D
 |   Map is 3D:              Yes

##### orig map
v.to.db -p t_hydro option=slope
Reading features...
 100%
cat|slope
-1|0
0 categories read from vector map (layer 1)
0 records updated/inserted (layer 1)

# draped map
v.to.db -p t_hydro_3D option=slope
Reading features...
 100%
cat|slope
-1|-6.16016389653397
0 categories read from vector map (layer 1)
0 records updated/inserted (layer 1)

# split map

v.to.db -p t_hydro_split option=slope
Reading features...
 100%
cat|slope
-1|-96.9755874160899
0 categories read from vector map (layer 1)
0 records updated/inserted (layer 1)

-> is that right? maybe there is some cat mess in the original map?

# 3D view:
v.info t_hydro_split | grep B
 |               B:     1085.64427482    T:     1698.36345331

-> reasonable

r.mapcalc "bottom = 1085"
nviz bottom vect=t_hydro_split

-> nice 3D vector lines

Markus