Help with 3D

Raster 2.5D Surfaces

This is the "typical" case, and most GRASS r.* modules work in this mode. The GIS layer is defined by north,south,east,west and the raster map's cell values represent the elevation.

• An introduction to raster processing in GRASS GIS

• Snippet from the Wikipedia definition for 2.5D: "The idea is that the program's canvas represents a normal 2D painting surface, but that the data structure that holds the pixel information is also able to store information regarding z-index (depth) as well as other information such as material settings, specularity, etc. With this data it is thus possible to simulate lighting, shadows, and so on."

• Horizontal (2D) region settings are handled by the g.region module. The third dimension is unbound.

• 2.5D rasters may be visualized in 3D with GRASS's NVIZ program, or as a shaded relief in 2D space with the r.shaded.relief module.

Screenshots

• GRASS 6.1 elevation screenshots

• Olympus Mons (Mars) topography

Example

(spearfish dataset)

g.region rast=elevation.10m
nviz elev=elevation.10m

Raster 3D Volumes (voxels)

Support for 3D rasters (voxels) is relatively new in GRASS and is primarily handled by the r3.* modules. A 3D raster can be thought of as a stack of 2D raster maps forming a 3D cube of data cells.

• An introduction to 3D raster processing in GRASS GIS

The third dimension need not be limited to "z" elevation! It can just as well represent the time dimension in a time series of 2.5D raster surfaces. It is possible to make a 4D visualization using the NVIZ software's animation panels and movie making facility.

Screenshots

• NVIZ Percipitaion isosurfaces above Slovakia

• Chesapeake Bay Nitrogen (4D) from NCSU GMS Lab (Mitasova et al.)

Region settings

Region settings are handled by the g.region module. Special 3D raster controls for g.region include:

-3 flag:  Prints 3D settings
      t   Value for the top edge
      b   Value for the bottom edge

   res3   3D grid resolution (north-south, east-west and top-bottom)
  tbres   Top-bottom grid resolution 3D

and

 rast3d   Set region to match this 3D raster map (both 2D and 3D values)

Tips

• From the mailing list: [1]

Examples

• Slovakia 3D precipitation

• Custom volume creation. Here is a small Octave (Matlab) script to create a text file suitable for loading as a 3D raster with r3.in.ascii. Here is the file that this script creates: vox_data50.txt.gz

% f(x,y,z) = x + (y^2 /2) + z^2
x= y= z= 50;

cd ~/grass/
fp = fopen('vox_data50.txt', 'wt')

fprintf(fp, 'north: %d\n', y);
fprintf(fp, 'south: 0\n');
fprintf(fp, 'east: %d\n', x);
fprintf(fp, 'west: 0\n');
fprintf(fp, 'top: %d\n', z);
fprintf(fp, 'bottom: 0\n');
fprintf(fp, 'rows: %d\n', y);
fprintf(fp, 'cols: %d\n', x);
fprintf(fp, 'levels: %d\n', z);

for dep = 1:z
  for row = 1:y
    for col = 1:x
       fprintf(fp, '%.4f', col + (row^2 /2) + dep^2 );
       if(col < 50)
          fprintf(fp, ' ');
       end
    end
    fprintf(fp, '\n');
  end
  disp(['dep ' num2str(dep)])
  fflush(stdout);
end
fclose(fp)

Load the data with:

r3.in.ascii in=vox_data50.txt out=vox50

display isosurfaces with:

g.region rast3d=vox50
nviz -q
Panel->Raster volumes
[New] vox50
Isosurface [Add] [New Constant] 500 [Accept]  
 (repeat for 1000,2000,3000)
Change main view height to 125.00 (manual entry)

extract a slice to a 2D raster map with:

g.region t=26.0 b=25.0
r3.to.rast in=vox50 out=vox50x

Vector 3D point data

Historical note: known as "site data" in GRASS GIS 5

In the "typical" case, "z" data is stored as a non-spatial attribute in a vector point map. In GRASS 6 it is possible to create 3D vector files, i.e. points exist in 3D space.

• An introduction to vector processing in GRASS GIS

Import x,y,z,... data as 3D with the v.in.ascii "-z" flag and "z=" column parameter:

v.in.ascii -z format=point cat=1 x=2 y=3 z=4

• It may be more convienent to import massive datasets (millions and millions of points) with r.in.xyz to create a 2.5D raster map.

Screenshot

3D points defining the hull of a ship, visualized using NVIZ

Example

TODO: import star guide and display using NVIZ, using magnitude column in database table for dynamic sizing. Requires a "Free" dataset, if you have any leads, please add links to this wiki page. Thanks.

Vector 3D polygons

In the "typical" case, "z" data is stored as a non-spatial attribute in a vector point map. In GRASS 6 it is possible to create 3D vector files, i.e. lines and polygons exist in 3D space. The topological nature of GRASS 6's vector engine means that roads can cross via a bridge without topologicly meeting.

• 3D feature types
  • face: The 3D equivalent of a boundary. These combine to form a volume surface (e.g. a teapot)
  • kernel: The 3D equivalent of a centroid (infrastructure for kernels is in place, but they are currently mostly unused)
  • volume: An enclosed 3D space. The 3D equivalent to GRASS's area vector feature, meaning an alias to a "face(s)+kernel" set. (some infrastructure and place-holders exist, but not much in the way of practical applications has been coded yet)

• An introduction to vector processing in GRASS GIS

• The GRASS ascii format specification

• The v.in.ascii -z flag must be used to create a 3D file

• The v.in.dxf or v.in.dwg modules can import 3D vector surfaces. DXF files are usually imported without georeference. To move into the geo-space of the rest of your data they must be projected with v.proj or translated with v.transform. If working with them alone, it is recommended to use a simple XY location.

• Visualization with NVIZ: must be placed on a raster surface or constant surface. It may be useful to set the transparency attribute of the raster surface to something unobtrusive.

Screenshots

• GRASS 6.1 screenshots

Example

pyramid: the following is an example of 3D vector surfaces in GRASS's standard ascii format. load with v.in.ascii.

# 3D Pyramid in GRASS:  (kernel currently unused?)
#  v.in.ascii in=pyramid.vasc out=pyramid_3d format=standard -nz
#  g.region n=150 s=-50 w=-50 e=150 res=10
#  r.mapcalc one=1
#  nviz elev=one vect=pyramid_3d
#
#  tip: in nviz try setting the raster surface transparency to 50%
#
F 5
 0 0 10
 0 100 10
 100 100 10
 100 0 10
 0 0 10
F 4
 0 0 10
 50 50 80
 100 0 10
 0 0 10
F 4
 0 100 10
 50 50 80
 0 0 10
 0 100 10
F 4
 100 100 10
 50 50 80
 0 100 10
 100 100 10
F 4
 100 0 10
 50 50 80
 100 100 10
 100 0 10
K 1 1
 40 50 40
 1 1

Visualization Tools

• NVIZ: GRASS's own visualization suite
  • Screenshots

• output to VTK & Paraview (r.out.vtk, r3.out.vtk, v.out.vtk)
• output to POVray (r.out.pov, v.out.pov)
• output to Vis5D (r3.out.v5d)