r.external - Links GDAL supported raster data as a pseudo GRASS raster map.

       Glynn Clements

r.external allows a user to link a GDAL supported raster file to a binary raster map layer, from any GDAL
       supported raster map format, with an optional title. The file is not  imported  but  just  registered  as
       GRASS raster map.

RGBOrthophotofromGeoTIFF
       # import of all channels (each channel will become a GRASS raster map):
       r.external input=/home/user/data/maps/059100.tif output=ortho
       g.region raster=ortho.3 -p
       d.rgb r=ortho.1 g=ortho.2 b=ortho.3
       r.composite r=ortho.1 g=ortho.2 b=ortho.3 output=ortho.rgb

   Processingworkflowwithoutdataimportandexport
       External  raster  maps  to be processed can be directly linked using r.external; likewise, results can be
       written out to standard raster formats with r.external.out (GDAL supported formats):
       # register GeoTIFF file to be used in current mapset:
       r.external input=terra_lst1km20030314.LST_Day.tif output=modis_celsius
       # define output directory for files resulting from GRASS calculation:
       r.external.out directory=$HOME/gisoutput/ format="GTiff"
       # perform GRASS calculation (here: extract pixels > 20 deg C)
       # this stores the output map directly as GeoTIFF:
       r.mapcalc "warm.tif = if(modis_celsius > 20.0, modis_celsius, null() )"
       # cease GDAL output connection and turn back to write GRASS raster files:
       r.external.out -r
       # now use the resulting file elsewhere
       gdalinfo $HOME/gisoutput/warm.tif

       raster, import, external

r.external  - Links GDAL supported raster data as a pseudo GRASS raster map.

       In  essence,  r.external  creates  a  read-only  link  to the original dataset which is only valid if the
       original dataset remains at the originally indicated directory and filename.

       GDAL-linked (r.external) maps do not have or use a NULL  bitmap,  hence  r.null  cannot  manipulate  them
       directly.   Here   NULL   cells   are   those   whose   value   matches   the   value   reported  by  the
       GDALGetRasterNoDataValue() function.  To apply the GDAL-linked the user need  to  either  create  a  MASK
       (e.g.  with r.mask) and then "apply" it using e.g. r.resample, or use r.mapcalc to create a copy with the
       appropriate categories changed to NULL (if() condition).

       GDAL Pages: https://gdal.org/

r.import,r.in.gdal,r.external.outv.import,v.in.ogr,v.external,v.external.out

       Available at: r.external source code (history)

       Accessed: Friday Apr 04 01:19:06 2025

       Main index | Raster index | Topics index | Keywords index | Graphical index | Full index

       © 2003-2025 GRASS Development Team, GRASS GIS 8.4.1 Reference Manual

GRASS 8.4.1                                                                                   r.external(1grass)

r.externalr.external--helpr.external  [-fojeahvtmr]   [input=name]    [source=name]   output=name   [band=integer]   [title=phrase]
       [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:-f
           List supported formats and exit

       -o
           Override projection check (use current project’s CRS)
           Assume that the dataset has the same coordinate reference system as the current project

       -j
           Perform projection check only and exit

       -e
           Extend region extents based on new dataset
           Also updates the default region if in the PERMANENT mapset

       -a
           Auto-adjustment for lat/lon
           Attempt to fix small precision errors in resolution and extents

       -h
           Flip horizontally

       -v
           Flip vertically

       -t
           List available bands including band type in dataset and exit
           Format: band number,type,projection check

       -m
           Read data range from metadata
           WARNING: metadata are sometimes approximations with wrong data range

       -r
           Create fast link without data range
           WARNING: some modules do not work correctly without known data range

       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:input=name
           Name of raster file to be linked

       source=name
           Name of non-file GDAL data source

       output=name[required]
           Name for output raster map

       band=integer
           Band to select (default is all bands)

       title=phrase
           Title for resultant raster map