DESCRIPTION
r.buildvrt builds a virtual raster (VRT) that is a mosaic of
the list of input raster maps. The purpose of such a VRT is to provide
fast access to small subsets of the VRT, also with multiple simultaneous
read requests.
NOTES
r.buildvrt creates a list of raster maps that can be
located in different mapsets. The output is a read-only link to
the original raster maps which is only valid if the original raster
maps remain in the originally indicated mapset. A VRT can also be built
from raster maps registered with r.external.
However, GRASS VRTs built from external registered data (see below)
are known to have performance issues.
Reading the whole VRT is slower than reading the equivalent single
raster map. Only reading small parts of the VRT provides a performance
benefit.
A GRASS virtual raster can be regarded as a simplified version of GDAL's
virtual raster format.
The GRASS equivalent is simpler because issues like nodata, projection,
resolution, resampling, masking are already handled by native GRASS
raster routines.
EXAMPLES
VRT from a DEM in the North Carolina sample dataset
In this exercise a low resolution DEM is resampled to a high
resolution DEM. This is subsequently cut into small tiles and
from that a virtual tile mosaik is created:
# set the computational region to elevation map
g.region raster=elev_state_500m -p
# enforce higher resolution
g.region res=50 -p
# resample the 500 meter DEM to 50 meter resolution
r.resamp.interp input=elev_state_500m output=elev_state_50m method=bilinear
# create tiles from resulting large 50 m elevation map
r.tile input=elev_state_50m output=elev_state_50m_tile_ width=1000 height=1000 overlap=0
# for convenience, dump list of tile names to a file
g.list type=raster pattern=elev_state_50m_tile_* output=tilelist.csv
# build a mosaik as VRT from tile list
r.buildvrt file=tilelist.csv output=elev_state_50m_vrt
KNOWN ISSUES
Users may experience significant performance degradation with virtual rasters built
with r.buildvrt over GDAL-linked (r.external) raster maps,
especially on slower file systems with latency like NFS. Performance degradation
may also occur on local file systems, but is usually less severe. For such use cases
consider using the GRASS GIS addon
r.buildvrt.gdal
or building GDAL VRTs, e.g. with gdalbuildvrt.
SEE ALSO
r.tile,
r.patch,
r.external
r.buildvrt.gdal
The equivalent GDAL utility
gdalbuildvrt
AUTHOR
Markus Metz
Sponsored by mundialis