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/
SEE ALSO
r.import,
r.in.gdal,
r.external.out
v.import,
v.in.ogr,
v.external,
v.external.out
AUTHOR
Glynn Clements
Last changed: $Date$