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

Snippet from the Wikipedia definition: "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.

It is possible to make a 4D (change with time) visualization using the NVIZ software's animation panels and movie making facility.

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 introdution to 3D rasters

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

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.

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

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)

• 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)