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.
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.r3.in.ascii and r3.in.bin supports generic x,y,z ASCII and binary array import. Alternatively, 3D rasters can be generated from 3D point vector data (v.to.rast3). Always the full map is imported. 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 composed of several 2D raster maps. 2D rasters are considered as slices in this case and merged into one 3D raster map (r.to.rast3). 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 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). r3.mapcalc. A 3D groundwater flow model is implemented in r3.gwflow. r3.to.rast). Cross sectional 2D raster map can be extracted from 3D raster map based on a 2D elevation map (r3.cross.rast). r3.stats and r3.univar. v.vol.rst). Results are 3D raster maps, however 2D raster maps can be also extracted. 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 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 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.
© 2003-2018 GRASS Development Team, GRASS GIS 7.5.svn Reference Manual