- Compresses and decompresses raster maps.
, map management
r.compress [-upg] map=name[,name,...] [--help] [--verbose] [--quiet] [--ui]
- Uncompress the map
- Print compression information and data type of input map(s)
- Print compression information in shell script style
- Print usage summary
- Verbose module output
- Quiet module output
- Force launching GUI dialog
- map=name[,name,...] [required]
- Name of existing raster map(s)
can be used to compress or decompress raster maps.
Additionally, it prints information about the compression method and
data type of the input raster map(s).
All raster maps (those imported for the first time and those newly
generated) are compressed by default using the ZLIB compression
method (see below). Related no data files (i.e.: NULL files), if
present, are compressed by default unless a specific environment
variable is set to explicitly disable NULL file compression
(GRASS_COMPRESS_NULLS, see below).
During compression or re-compression, r.compress compresses
raster maps using the method specified by means of the environment
variable GRASS_COMPRESSOR. The default compression method is
ZLIB's "deflate" algorithm (LZ77-based). Raster maps that contain very
little information (such as boundary, geology, soils and land use maps)
can be greatly reduced in size. Some raster maps are shrunk to roughly
1% of their original sizes.
All newly generated raster maps are automatically stored as compressed
data with varying methods depending on the raster format (i.e.,
CELL: integer; FCELL: single precision; DCELL: double precision; see
below). All GRASS GIS modules are able to read both compressed and
uncompressed raster maps.
Raster maps that are already compressed might be compressed again,
either by setting a different method with GRASS_COMPRESSOR
(supported methods: RLE, ZLIB, LZ4, BZIP2, ZSTD) or, for the case of
ZLIB compression, by changing the compression level with the
environment variable GRASS_ZLIB_LEVEL.
Compressed raster maps may be decompressed using r.compress
with the -u flag. If a raster map was already decompressed and
the -u flag is set, the module simply informs the user that the
map is already decompressed and exits.
Information about the compression method and data type of the input
raster map(s) can be printed in shell style with the -g flag. In
this case, the module prints to stdout one line per input map
with the fields "input map name", "data type",
"name of data compression method", "NULL file
compression" separated by the pipe character. NULL file
compression is indicated with "YES" or "NO".
- INTEGER map (CELL data type): a raster map of INTEGER type (whole
- FLOAT map (FCELL data type): a raster map of FLOAT type (4 bytes,
7-9 digits precision)
- DOUBLE map (DCELL data type): a raster map of DOUBLE type (8 bytes,
15-17 digits precision)
- NULL: represents "no data" in raster maps; to be distinguished from
0 (zero) data value
The following compression methods are available (set by
- NONE (uncompressed)
- RLE (generic Run-Length Encoding of single bytes; deprecated)
- ZLIB (DEFLATE, good speed and compression)
- with zlib compression levels (export GRASS_ZLIB_LEVEL=X): -1..9
(-1 is default which corresponds to ZLIB level 6)
- note: export GRASS_ZLIB_LEVEL=0 is equal to copying the data
as-is from source to destination
- LZ4 (fastest, low compression)
- BZIP2 (slowest, high compression)
- ZSTD (compared to ZLIB, faster and higher compression,
much faster decompression - default compression)
Important: the NULL file compression can be turned off with
. Raster maps with NULL file
compression can only be opened with GRASS GIS 7.2.0 or later. NULL file
compression for a particular raster map can be managed with r.null -z
The NULL file compression is using the LZ4 method as being the best compromise
between speed and compression rate.
All GRASS GIS raster map types are by default ZLIB compressed, i.e. using
ZLIB's deflate algorithm. Through the environment variable
the compression method can be set to RLE, ZLIB,
LZ4, BZIP2, or ZSTD.
Integer (CELL type) raster maps can be compressed with RLE if
the environment variable GRASS_COMPRESSOR exists and is set to
RLE. However, this is not recommended.
Floating point (FCELL, DCELL) raster maps never use RLE compression;
they are either compressed with ZLIB, LZ4, BZIP2, ZSTD or are uncompressed.
- DEPRECATED Run-Length Encoding, poor compression ratio but
fast. It is kept for backwards compatibility to read raster maps
created with GRASS 6. It is only used for raster maps of type CELL.
FCELL and DCELL maps are never and have never been compressed with RLE.
- ZLIB's deflate is the default compression method for all raster
maps, if ZSTD is not available. GRASS GIS 7 uses by default 1 as ZLIB
compression level which is the best compromise between speed and
compression ratio, also when compared to other available compression
methods. Valid levels are in the range [1, 9] and can be set with the
environment variable GRASS_ZLIB_LEVEL.
- LZ4 is a very fast compression method, about as fast as no
compression. Decompression is also very fast. The compression ratio is
generally higher than for RLE but worse than for ZLIB. LZ4 is
recommended if disk space is not a limiting factor.
- BZIP2 can provide compression ratios much higher than the other
methods, but only for large raster maps (> 10000 columns). For large
raster maps, disk space consumption can be reduced by 30 - 50% when
using BZIP2 instead of ZLIB's deflate. BZIP2 is the slowest compression
and decompression method. However, if reading from / writing to a
storage device is the limiting factor, BZIP2 compression can speed up
raster map processing. Be aware that for smaller raster maps, BZIP2
compression ratio can be worse than other compression methods.
- ZSTD (Zstandard) provides compression ratios higher than ZLIB but
lower than BZIP2 (for large data). ZSTD compresses up to 4x faster than
ZLIB, and usually decompresses 6x faster than ZLIB. ZSTD is the
default compression method if available.
The used compression method is encoded with numbers. In the internal
file, the value for "compressed" is 1 for RLE, 2
for ZLIB, 3 for LZ4, 4 for BZIP2, and 5 for ZSTD.
Obviously, decompression is controlled by the raster map's compression,
not by the environment variable.
Conceptually, a raster data file consists of rows of cells, with each
row containing the same number of cells. A cell consists of one or more
bytes. For CELL maps, the number of bytes per cell depends on the
category values stored in the cell. Category values in the range 0-255
require 1 byte per cell, while category values in the range 256-65535
require 2 bytes, and category values in the range above 65535 require 3
(or more) bytes per cell.
FCELL maps always have 4 bytes per cell and DCELL maps always have 8
bytes per cell.
Since GRASS GIS 7.0.0, the default compression method for
Integer (CELL) raster maps is ZLIB and no longer RLE.
If the environment variable GRASS_ZLIB_LEVEL
its value can be parsed as an integer, it determines the compression
level used when newly generated raster maps are compressed using ZLIB
compression. This applies to all raster map types (CELL, FCELL, DCELL).
If the variable does not exist, or the value cannot be parsed as an
integer, ZLIB's compression level 1 will be used.
Printing of current compression state:
r.compress compressed_no -p
<compressed_no> (method 0: NONE). Data type: <CELL>
Applying RLE compression to a copy of the uncompressed map (not recommended!):
# compression of map using the deprecated RLE compression
export GRASS_COMPRESSOR=RLE # RLE
r.compress compressed_RLE -p
<compressed_RLE> is compressed (method 1: RLE). Data type: <CELL>
Applying ZLIB compression to a copy of the uncompressed map:
# compression of map using ZLIB compression
export GRASS_COMPRESSOR=ZLIB # ZLIB
r.compress compressed_ZLIB -p
<compressed_ZLIB> is compressed (method 2: ZLIB). Data type: <CELL>
Applying BZIP2 compression to a copy of the ZLIB-compressed map:
# compression of map using BZIP2 compression
export GRASS_COMPRESSOR=BZIP2 # BZIP2
r.compress compressed_BZIP2 -p
<compressed_BZIP2> is compressed (method 4: BZIP2). Data type: <CELL>
Applying ZSTD compression to a copy of the BZIP2-compressed map:
# compression of map using ZSTD compression
export GRASS_COMPRESSOR=ZSTD # ZSTD
r.compress compressed_ZSTD -p
<compressed_ZSTD> is compressed (method 5: ZSTD). Data type: <CELL>
James Westervelt and Michael Shapiro, U.S. Army Construction Engineering Research Laboratory
Last changed: $Date: 2018-06-10 12:16:22 -0700 (Sun, 10 Jun 2018) $
Available at: r.compress source code (history)
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GRASS Development Team,
GRASS GIS 7.5.svn Reference Manual