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.
- 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
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.
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
- 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
- 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.
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)
- 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
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
- 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)