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r.external.1grass - Man Page

Links GDAL supported raster data as a pseudo GRASS raster map.


raster, import, external


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



List supported formats and exit


Override projection check (use current location’s projection)
Assume that the dataset has same projection as the current location


Perform projection check only and exit


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


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


Flip horizontally


Flip vertically


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


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


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


Allow output files to overwrite existing files


Print usage summary


Verbose module output


Quiet module output


Force launching GUI dialog



Name of raster file to be linked


Name of non-file GDAL data source

output=name [required]

Name for output raster map


Band to select (default is all bands)


Title for resultant raster map


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.


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.

NULL data handling

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).


RGB Orthophoto from GeoTIFF

# 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

Processing workflow without data import and export

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


GDAL Pages: http://www.gdal.org/

See Also

r.import, r.in.gdal, r.external.out

v.import, v.in.ogr, v.external, v.external.out


Glynn Clements

Source Code

Available at: r.external source code (history)

Accessed: Tuesday Mar 19 11:01:58 2024

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