## DESCRIPTION

r.out.gdal allows a user to export a GRASS raster map layer into any GDAL supported raster map format. If a GRASS raster map is exported for a particular application, the application's native format would be preferable. GeoTIFF is supported by a wide range of applications (see also NOTES on GeoTIFF below).

To specify multiple creation options use a comma separated list (createopt="TFW=YES,COMPRESS=DEFLATE").

For possible createopt and metaopt parameters please consult the individual supported formats pages on the GDAL website. The createopt parameter may be used to create TFW or World files ("TFW=YES","WORLDFILE=ON").

r.out.gdal also supports the export of multiband rasters as a group, when the imagery group's name is entered as input. (created imagery groups with the i.group module)

As with most GRASS raster modules, the current region extents and region resolution are used, and a raster mask is respected if present. Use g.region's "align=", or "raster=" options if you need to realign the region settings to match the original map's before export.

## SUPPORTED RASTER FORMATS

The set of supported raster formats written by r.out.gdal depends on the local GDAL installation, printed with the -l flag. Available may be (incomplete list):

      AAIGrid: Arc/Info ASCII Grid
      BMP: MS Windows Device Independent Bitmap
      BSB: Maptech BSB Nautical Charts
      DTED: DTED Elevation Raster
      ELAS: ELAS
      ENVI: ENVI .hdr Labelled
      FIT: FIT Image
      GIF: Graphics Interchange Format (.gif)
      GTiff: GeoTIFF
      HDF4Image: HDF4 Dataset
      HFA: Erdas Imagine Images (.img)
      JPEG2000: JPEG-2000 part 1 (ISO/IEC 15444-1)
      JPEG: JPEG JFIF
      MEM: In Memory Raster
      MFF2: Atlantis MFF2 (HKV) Raster
      MFF: Atlantis MFF Raster
      NITF: National Imagery Transmission Format
      PAux: PCI .aux Labelled
      PCIDSK: PCIDSK Database File
      PNG: Portable Network Graphics
      PNM: Portable Pixmap Format (netpbm)
      VRT: Virtual Raster
      XPM: X11 PixMap Format

## NOTES

Out of the GDAL data types, the closest match for GRASS CELL, FCELL and DCELL rasters are respectively Int32, Float32 and Float64. These are not exact equivalents, but they will preserve the maximum possible data range and number of decimal places for each respective GRASS raster data type. Please keep in mind that not all CELL rasters will require Int32 - e.g., 0-255 CELL raster are covered by the Byte type as well. Moreover, some GDAL-supported formats do not support all the data types possible in GDAL and GRASS. Use r.info to check the data type and range for your GRASS raster, refer to specific format documentation (on the GDAL website), format vendor's documentation, and e.g. the Wikipedia article Typical boundaries of primitive integral types for details.

### Ranges of GDAL data types

      GDAL data type	       minimum  	maximum

      Byte  			     0  	    255
      UInt16			     0  	 65,535
      Int16, CInt16 	       -32,768  	 32,767
      UInt32			     0    4,294,967,295
      Int32, CInt32 	-2,147,483,648    2,147,483,647
      Float32, CFloat32	       -3.4E38  	 3.4E38
      Float64, CFloat64	     -1.79E308         1.79E308

If there is a need to keep file sizes small, use the simplest data type covering the data range of the raster(s) to be exported, e.g., if suitable use Byte rather than UInt16; use Int16 rather than Int32; or use Float32 rather than Float64. In addition, the COMPRESS createopt used can have a very large impact on the size of the output file.

Some software may not recognize all of the compression methods available for a given file format, and certain compression methods may only be supported for certain data types (depends on vendor and version).

By default, r.out.gdal identifies suitable settings for nodata and data type, if not given by the user. If however the export settings are set such that data loss would occur in the output file (i.e, due to the particular choice of data type and/or file type), the normal behaviour of r.out.gdal in this case would be to issue an error message describing the problem and exit without exporting. However, depending on the size of the data, tests to check for data loss may take some time. For performance reasons, these tests can be skipped if the -fb> flag, the nodatab> option and the typeb> option are all specified. The assumption here is that the user has made a deliberate choice about these settings and considered the potential risk of data loss when forcing the export with the -fb> flag.

r.out.gdal exports may appear all black or gray on initial display in other GIS software. This is not a bug of r.out.gdal, but often caused by the default color table assigned by that software. The default color table may be grayscale covering the whole range of possible values which is very large for e.g. Int32 or Float32. E.g. stretching the color table to actual min/max would help (sometimes under symbology).

### Adding overviews to speed up map display in other software

Adding overviews with `gdaladdo` after exporting can speed up display. The overviews are created internally within the exported file. The amount of levels (power-of-two factors) are controlled with the overviews parameter. The higher the overview level defined by the user the more lower resolution internal overviews are added Note that other software might create their own overviews, ignoring existing overviews.

### GeoTIFF caveats

GeoTIFF exports can only be displayed by standard image viewers if the GDAL data type was set to Byte and the GeoTIFF contains either one or three bands. All other data types and numbers of bands can be properly read with GIS software only. Although GeoTIFF files usually have a .tif extension, these files are not necessarily images but first of all spatial raster datasets, e.g. land cover or elevation.

When writing out multi-band GeoTIFF images for users of ESRI software or ImageMagick, the interleaving mode should be set to "pixel" using createopt="INTERLEAVE=PIXEL". BAND interleaving is slightly more efficient, but not supported by some applications. This issue only arises when writing out multi-band imagery groups.

Classic TIFF format supports only files with up to 4GB. Files that exceed this limit (in compressed or uncompressed form) need to be exported with createopt="BIGTIFF=YES". BIGTIFF is available if GDAL is built with libtiff &= 4.0.

### Improving GeoTIFF compatibility

To create a GeoTIFF that is highly compatible with various other GIS software packages, it is recommended to keep the GeoTIFF file as simple as possible. You will have to experiment with which options your software is compatible with, as this varies widely between vendors and versions. Long term, the less metadata you have to remove the more self-documenting (and useful) the dataset will be.

Here are some things to try:

- Create a World file with `createopt="TFW=YES"`.
- Do not use GeoTIFF internal compression. Other GIS software often supports only a subset of the available compression methods with the supported methods differing between GIS software packages. Unfortunately this means the output image can be rather huge, but the file can be compressed with software like `zip`, `gnuzip`, or `bzip2`.
- Skip exporting the color table. Color tables are not always properly rendered, particularly for type UInt16, and the GeoTIFF file can appear completely black. If you are lucky the problematic software package has a method to reset the color table and assign a new color table (sometimes called symbology).
- Keep metadata simple with `createopt="PROFILE=GeoTIFF"` or `createopt="PROFILE=BASELINE"`. With BASELINE no GDAL or GeoTIFF tags will be written and a World file is required (createopt="TFW=YES").

### Offset/scale parameters

Offset is only relevant if not zero. Scale is only relevant if not 1.

## EXAMPLES

### Export the integer raster basin_50K map to GeoTIFF format

See also GeoTIFF format description (GDAL):

    g.region raster=basin_50K -p
    r.out.gdal input=basin_50K output=basin_50K.tif

### Export the integer raster landclass96 map to Cloud Optimized GeoTIFF format

See also Cloud Optimized GeoTIFF (COG) format description (GDAL):

    g.region -p raster=landclass96
    r.out.gdal -fmt input=landclass96 output=landclass96.tif format=COG overviews=4

### Export a DCELL raster map in GeoTIFF format suitable for ESRI software

    g.region raster=elevation -p
    r.out.gdal in=elevation output=elevation.tif createopt="PROFILE=GeoTIFF,TFW=YES"

### Export a raster map in "Deflate" compressed GeoTIFF format

    g.region raster=elevation -p
    r.out.gdal in=elevation output=elevation.tif createopt="COMPRESS=DEFLATE"

### Export a large raster map in LZW compressed (Big) GeoTIFF format

    # integer map export
    g.region raster=zipcodes -p
    # Using PREDICTOR 2 for integer maps can further reduce file size
    r.out.gdal in=zipcodes output=zipcodes.tif createopt="COMPRESS=LZW,PREDICTOR=2,BIGTIFF=YES"

    # floating point map export
    g.region raster=elevation -p
    # Using PREDICTOR 3 for floating point data can further reduce file size
    r.out.gdal in=elevation output=elevation.tif createopt="COMPRESS=LZW,PREDICTOR=3,BIGTIFF=YES"

### Export a raster map with internal overview in "Deflate" compressed GeoTIFF format

    g.region raster=elevation -p
    # overviews=5 corresponds to 'gdaladdo ... 2 4 8 16 32'
    r.out.gdal in=elevation output=elevation.tif createopt="COMPRESS=DEFLATE" overviews=5

### Export R,G,B imagery bands in GeoTIFF format suitable for ESRI software

    i.group group=nc_landsat_rgb input=lsat7_2002_30,lsat7_2002_20,lsat7_2002_10
    g.region raster=lsat7_2002_30 -p
    r.out.gdal in=nc_landsat_rgb output=nc_landsat_rgb.tif type=Byte \
      createopt="PROFILE=GeoTIFF,INTERLEAVE=PIXEL,TFW=YES"

### Export group of image maps as multi-band file

    g.list group
    i.group group=tm7 subgroup=tm7 input=tm7_10,tm7_20,tm7_30,tm7_40,tm7_50,tm7_60,tm7_70
    i.group -l tm7
    g.region raster=tm7_10 -p
    r.out.gdal tm7 output=lsat_multiband.tif
    gdalinfo lsat_multiband.tif

### Export RGB with alpha channel that encodes NULL cells

When exporting exporting RGB data rather than GIS data for Web applications or generally the scope of visualization, the alpha channel is of use. Here the export type is commonly the Byte data type.

When exporting data with r.out.gdal, assigning a nodata value (specific parameter of the module) means that any band values equal to this nodata value will be interpreted as nodata. Using an additional alpha channel means that all pixels with an alpha value of 0 are transparent. The alpha channel thus represents per-pixel encoding of nodata, just like the GRASS raster mask or per-raster null file. That means when using an alpha channel, you do not need to "free up" any particular value, but you can use any value you like to replace NULL cells, as long as the value can be represented by the Byte data type. It does not matter if that value is already present in any of the input bands.

Hence for "visual-only" RGB data export it is needed to create an additional alpha channel that encodes all NULL cells and in the RGB bands to be exported replace NULL cells with some value in the range 0-255. For example:

    # for simplicity variables are used
    RMAP="lsat7_2000_30"
    GMAP="lsat7_2000_20"
    BMAP="lsat7_2000_10"

    OUTNAME="lsat7_2000_RGBA.tif"

    # extract alpha
    r.mapcalc "out_a = if(isnull($RMAP) || isnull($GMAP) || isnull($BMAP), 0, 255)"

    # replace NULL cells with a valid value, extract colors

    # exporting 8 bit RGB data, not GIS data, therefore the `#` operator:
    r.mapcalc "out_r = if(isnull($RMAP), 0, #$RMAP)"
    r.mapcalc "out_g = if(isnull($GMAP), 0, #$GMAP)"
    r.mapcalc "out_b = if(isnull($BMAP), 0, #$BMAP)"

    # create group for export
    i.group group=out_rgba input=out_r,out_g,out_b,out_a

    # remove any mask because this works only if there are
    # no NULL cells in the bands to be exported
    r.mask -r

    # export the group:
    # add PROFILE=BASELINE to createopt to produce a standard TIFF file
    # without any GTiff extensions
    r.out.gdal input=out_rgba output=$OUTNAME -cm createopt="PHOTOMETRIC=RGB,ALPHA=YES"
    gdalinfo $OUTNAME

The resulting GeoTIFF file can be used e.g. for Web server applications.

### Export the floating point raster elevation map to ERDAS/IMG format

See also Erdas Imagine .img format description (GDAL):

    g.region raster=elevation -p
    r.out.gdal input=elevation output=elelevation.img format=HFA type=Float32

### Export raster map with offset/scale assigned

An offset value can be assigned to output raster data by offset parameter. Similarly a scale value can be assigned by scale parameter.

    # produce raster map
    g.region n=100 s=0 e=100 w=0 res=1
    r.random.cells output=random distance=1.0
    r.info -r random

    # export raster data with offset/scale assigned
    r.out.gdal input=random output=random.tif offset=100 scale=0.01

Check result by gdalinfo command line tool:

    gdalinfo random.tif
    ...
    Band 1 Block=100x40 Type=UInt16, ColorInterp=Palette
      Description = random
        Computed Min/Max=1.000,3661.000
      NoData Value=65535
      Offset: 100,   Scale:0.01
    ...

## GDAL RELATED ERROR MESSAGES

- "ERROR 6: SetColorInterpretation() not supported for this dataset.": This may indicate that the color table was not written properly. But usually it will be correct and the message can be ignored.
- "ERROR 6: SetNoDataValue() not supported for this dataset.": The selected output format does not support "no data". It is recommended to use a different output format if your data contains NULLs.
- "Warning 1: Lost metadata writing to GeoTIFF ... too large to fit in tag.": The color table metadata may be too large. It is recommended to simplify or not write the color table, or use a different output format or the flags -c and -m.

## REFERENCES

GDAL Pages: https://gdal.org

## SEE ALSO

GDAL supported raster formats and GDAL supported vector formats

r.out.ascii, r.out.bin, r.out.mat, r.out.png, r.out.ppm, r.pack

## AUTHORS

Vytautas Vebra (oliver4grass at gmail.com)
Markus Metz (improved nodata logic)