3D raster data in GRASS GIS
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 volume.
3D rasters are designed to support representations of
trivariate continuous fields.
The vertical dimension supports spatial and temporal units.
Hence space time 3D raster with different temporal resolutions
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 import 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
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 refer 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 mapping 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 confusing 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 resolution.
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
The module family prefix is
Occasionally, 3D raster related things can be
referred differently, for example according to a programming language 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 horizontal 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 isosuface 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.
Import from external files
The modules r3.in.ascii
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 convert 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
Conversion from 3D vector points
3D rasters can be generated from 3D point vector data
). Always 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)
a 3D raster can be composed of several 2D raster maps (stack of maps).
2D rasters are considered as slices in this case and
merged into one 3D raster map (r.to.rast3
GRASS GIS 3D raster map processing is always performed in the current 3D region
settings (see g.region
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 resampling 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
Powerful 3D raster map algebra is implemented in
A 3D groundwater flow model is implemented in r3.gwflow
Slices from a 3D raster map can be converted to a 2D raster map
Cross sectional 2D raster map can be extracted from 3D raster map based
on a 2D elevation map (r3.cross.rast
3D raster statistics can be calculated with r3.stats
From 3D vector points, GRASS 3D raster maps can be interpolated
Results are 3D raster maps, however 2D raster maps can be also extracted.
The modules r3.out.ascii
support the export of
3D raster maps as ASCII or binary files. The output of these modules
can be imported 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.
GRASS GIS can be used for visualization of 3D rasters, however
it has also tools to easily export the data into other visualization
GRASS GIS 3D raster maps can be exported to VTK
VTK files can be visualized with the
Moreover, GRASS GIS 2D raster maps can be exported to VTK with
and GRASS GIS vector maps can be exported to VTK with
Alternatively, GRASS 3D raster maps can be imported and exported from/to
3D raster's single-precision data type is most often called "FCELL" or "float",
and the double-precision one "DCELL" or "double".
Available at: 3D raster data in GRASS GIS source code (history)
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GRASS Development Team,
GRASS GIS 7.4.2svn Reference Manual