[GRASS-user] Problems with NVIZ Volume (3dGRID) Visualization !

[suenia]

                Hello together !
  I have problems with NVIZ Volume visualization !
 Here I made an example generating a simple 3dgrid volume assuming subterranean data and the failing of visualizing it. I would really appreciate some help !  
      Here the short workflow:
#Generating three surfaces which will model the volume.
GRASS 6.2.2 (4326):~ > r.mapcalc dem.t=0
GRASS 6.2.2 (4326):~ > r.mapcalc dem.m=-10
GRASS 6.2.2 (4326):~ > r.mapcalc dem.b=-20
                          #My region settings
GRASS 6.2.2 (4326):~ > g.region -p3
projection: 3 (Latitude-Longitude)
zone:       0
datum:      wgs84
ellipsoid:  a=6378137 es=0.006694379990141316
north:      24:56:24N
south:      12:54S
west:       75:52:12E
east:       114:17:24E
top:        0.00000000
bottom:     -20.00000000
nsres:      0:59:44.842105
nsres3:     0:59:44.842105
ewres:      1:00:39.789474
ewres3:     1:00:39.789474
tbres:      1
rows:       38
rows3:      38
cols:       38
cols3:      38
depths:     20
cells:      1444
3dcells:    28880

 
  #Generating the 3d grid volume
GRASS 6.2.2 (4326):~ > r.to.rast3 input=dem.b,dem.m,dem.t output=dem –o  
    #Visualizing it!
GRASS 6.2.2 (4326):~ > nviz volume=dem elevation=dem.t
 
 When visualizing the 3dgrid volume dem, together with the raster surfaces, only outlines in form of a cube volume are drawn whenever navigating in space !!! else nothing is seen but the raster surfaces.  
  When adding a new constant value, for example -10, to the raster volume, an associated unique colored surface at the bottom of the cube is observable.
  So, I´m not sure about what could be the problem or my mistake in the settings selection.
  Furthermore I would like to know how to change g.region res3, respectivly tbres for latitude longitude locations, when I try to change res3<0:59:44.842105 or tbres my region get's destroyed, definitely.
For what stands the default value 1 of tbres ? 
  
 Could somebody offer more information about vertical resolution specifications for latitude longitude locations.
      That works : Slovakia3d can be visualised by adding new constant value for the raster volume. 

Many thanks !
Suena
 
       
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