raster3dintro(1grass) GRASS GIS User's Manual raster3dintro(1grass)
3D raster data in GRASS GIS
3D raster maps in general
GRASS GIS is one of the few GIS software packages with 3D raster data
support. Data are stored as a 3D raster with 3D cells of a given vol-
ume. 3D rasters are designed to support representations of trivariate
continuous fields. The vertical dimension supports spatial and tempo-
ral units. Hence space time 3D raster with different temporal resolu-
tions can be created and processed.
GRASS GIS 3D raster maps use the same coordinate system as 2D raster
maps (row count from north to south) with an additional z dimension
(depth) counting from bottom to top. The upper left corner (NW) is the
origin. 3D rasters are stored using a tile cache based approach. This
allows arbitrary read and write operations in the created 3D raster.
The size of the tiles can be specified at import time with a given im-
port module such as r3.in.ascii or the data can be retiled using
r3.retile after import or creation.
The 3D raster map coordinate system and the internal tile layout of
the RASTER3D library
Terminology and naming
In GRASS GIS terminology, continuous 3D data represented by regular
grid or lattice is called 3D raster map. 3D raster map works in 3D in
the same was as (2D) raster map in 2D, so it is called the same except
for the additional 3D. Some literature or other software may use terms
such as 3D grid, 3D lattice, 3D matrix, 3D array, volume, voxel, voxel
model, or voxel cube. Note that terms volume and volumetric often re-
fer to measuring volume (amount) of some substance which may or may not
be related to 3D rasters.
Note that GRASS GIS uses the term 3D raster map or just 3D raster for
short, rather than 3D raster layer because term map emphasizes the map-
ping of positions to values which is the purpose of 3D raster map (in
mathematics, map or mapping is close to a term function) On the other
hand, the term layer emphasizes overlaying or stacking up. The former
is not the only operation done with data and the latter could be con-
fusing in case of 3D raster data.
3D raster map is divided into cells in the same way as the (2D) raster
map. A cell is a cube or a (rectangular) cuboid depending on the reso-
lution. The resolution influences volume of one cell. Some literature
or other software may use terms such as volume, volume unit, volumetric
pixel, volume pixel, or voxel. Note that voxel can be sometimes used
to refer to a whole 3D raster and that for example in 3D computer
graphics, voxel can denote object with some complicated shape.
Type of map and element name in GRASS GIS is called raster_3d. The
module family prefix is r3. Occasionally, 3D raster related things can
be referred differently, for example according to a programming lan-
guage standards. This might be the case of some functions or classes
in Python.
In GRASS GIS 3D rasters as stored in tiles which are hidden from user
most of the time. When analyzing or visualizing 3D rasters user can
create slices or cross sections. Slices can be horizontal, vertical, or
general plains going through a 3D raster. Slices, especially the hori-
zontal ones, may be called layers in some literature or some other
software. Cross sections are general functions, e.g. defined by 2D
raster, going through a 3D raster. Another often used term is an isos-
uface which has the same relation to 3D raster as contour (isoline) to
a 2D raster. An isosurface is a surface that represent places with a
constant value.
When 3D raster is used in the way that vertical dimension represents
time 3D raster can be referred to as space time cubes (STC) or space
time cube 3D raster. Some literature may also use space time voxel
cube, space time voxel model or some other combination.
3D raster import
Import from external files
The modules r3.in.ascii and r3.in.bin supports generic x,y,z ASCII and
binary array import.
In case of CSV tables, the modules v.in.ascii (using the -z flag) may
be a choice to first import the 3D points as vector points and the con-
vert them to 3D raster (see below).
Import of 3D (LiDAR) points and their statistics can be done using
r3.in.lidar for LiDAR data and r3.in.xyz for CSV and other ASCII text
formats.
Conversion from 3D vector points
3D rasters can be generated from 3D point vector data (v.to.rast3). Al-
ways the full map is imported.
Conversion from 2D raster maps
3D raster can also be created based on 2D elevation map(s) and value
raster map(s) (r.to.rast3elev). Alternatively, a 3D raster can be com-
posed of several 2D raster maps (stack of maps). 2D rasters are con-
sidered as slices in this case and merged into one 3D raster map
(r.to.rast3).
3D region settings and 3D MASK
GRASS GIS 3D raster map processing is always performed in the current
3D region settings (see g.region, -p3 flags), i.e. the current region
extent, vertical extent and current 3D resolution are used. If the 3D
resolution differs from that of the input raster map(s), on-the-fly re-
sampling is performed (nearest neighbor resampling). If this is not
desired, the input map(s) has/have to be reinterpolated beforehand with
one of the dedicated modules. Masks can be set (r3.mask).
3D raster analyses and operations
Powerful 3D raster map algebra is implemented in r3.mapcalc. A 3D
groundwater flow model is implemented in r3.gwflow.
3D raster conversion to vector or 2D raster maps
Slices from a 3D raster map can be converted to a 2D raster map
(r3.to.rast). Cross sectional 2D raster map can be extracted from 3D
raster map based on a 2D elevation map (r3.cross.rast).
3D raster statistics
3D raster statistics can be calculated with r3.stats and r3.univar.
3D raster interpolation
From 3D vector points, GRASS 3D raster maps can be interpolated
(v.vol.rst). Results are 3D raster maps, however 2D raster maps can be
also extracted.
3D raster export
The modules r3.out.ascii and r3.out.bin support the export of 3D raster
maps as ASCII or binary files. The output of these modules can be im-
ported with the corresponding import modules noted above.
NetCDF export of 3D raster maps can be performed using the module
r3.out.netcdf. It supports 3D raster maps with spatial dimensions and
temporal (vertical) dimension.
Working with 3D visualization software
GRASS GIS can be used for visualization of 3D rasters, however it has
also tools to easily export the data into other visualization packages.
GRASS GIS 3D raster maps can be exported to VTK using r3.out.vtk. VTK
files can be visualized with the VTK Toolkit, Paraview and MayaVi.
Moreover, GRASS GIS 2D raster maps can be exported to VTK with
r.out.vtk and GRASS GIS vector maps can be exported to VTK with
v.out.vtk.
Alternatively, GRASS 3D raster maps can be imported and exported
from/to Vis5D (r3.in.v5d, r3.out.v5d).
3D raster data types
3D raster’s single-precision data type is most often called "FCELL" or
"float", and the double-precision one "DCELL" or "double".
See also
• Introduction into raster data processing
• Introduction into vector data processing
• Introduction into image processing
• Introduction into temporal data processing
• Projections and spatial transformations
• wxGUI 3D View Mode
• m.nviz.image
SOURCE CODE
Available at: 3D raster data in GRASS GIS source code (history)
Accessed: unknown
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GRASS 7.8.7 raster3dintro(1grass)
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