## 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: $Date$