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 /home/user/data/maps/059100.tif out=ortho
g.region rast=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
REFERENCES
GDAL Pages: http://www.gdal.org/
SEE ALSO
r.in.gdal,
r.external.out
v.in.ogr,
v.external,
v.external.out
AUTHOR
Glynn Clements
Last changed: $Date$