r.externalgrass - Online in the Cloud

PROGRAM:

NAME

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

KEYWORDS

raster, import, external

SYNOPSIS

r.external
r.external --help
r.external [-foehv] [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 location’s projection)
Assume that the dataset has same projection as the current location

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

-h
Flip horizontally

-v
Flip vertically

--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

DESCRIPTION

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.

NOTES

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).

EXAMPLES

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

REFERENCES

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

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