r.external - GRASS GIS manual

NAME

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

KEYWORDS

SYNOPSIS

r.external

r.external --help

r.external [-fojeahvtr] [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
-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
-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

DESCRIPTION
NOTES
NULL data handling
EXAMPLES
- RGB Orthophoto from GeoTIFF
- Processing workflow without data import and export
REFERENCES
SEE ALSO
AUTHOR

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/

AUTHOR

Glynn Clements

SOURCE CODE

Available at: r.external source code (history)

Latest change: Wednesday Jan 27 21:15:26 2021 in commit: 2cf7e08b679327053f39d01060dd6a9e08e34f71