Ubuntu Manpage: r.patch - Creates a composite raster map layer by using known category values from one

NAME

       r.patch   -  Creates  a composite raster map layer by using known category values from one
       (or more) map layer(s) to fill in areas of "no data" in another map layer.

KEYWORDS

       raster, geometry, mosaicking, merge, patching, aggregation, series, parallel

SYNOPSIS

       r.patch
       r.patch --help
       r.patch [-zs] input=name[,name,...] output=name  [nprocs=integer]   [memory=memory in  MB]
       [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       -z
           Use zero (0) for transparency instead of NULL

       -s
           Do not create color and category files

       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       input=name[,name,...] [required]
           Name of raster maps to be patched together

       output=name [required]
           Name for resultant raster map

       nprocs=integer
           Number of threads for parallel computing
           Default: 1

       memory=memory in MB
           Maximum memory to be used (in MB)
           Cache size for raster rows
           Default: 300

DESCRIPTION

       The  GRASS  program  r.patch  allows  the  user  to  build  a  new raster map the size and
       resolution of the current region by assigning known data values from input raster maps  to
       the cells in this region.
       In case of overlapping input raster maps this is done by filling in "no data" cells, those
       that do not yet contain data, contain NULL data, or, optionally contain 0 data,  with  the
       data from the first input map.  Once this is done the remaining holes are filled in by the
       next input map, and so on.
       In case of adjacent input raster maps the output map contains the map mosaic.

       Hence this command is useful for

           •   making a composite raster map layer from two or more adjacent map layers,

           •   for filling in "holes" in a raster map layer’s data (e.g.,  in  digital  elevation
               data), or

           •   for updating an older map layer with more recent data.
       The current geographic region definition and mask settings are respected.

       Figure: Result of patching of two raster maps containing NULLs using the default settings.

   Stacking order
       The  first name listed in the string input=name,name,name,... is the name of the first map
       whose data values will be used to fill in cells in the current region.  Then,  the  second
       through  the last input maps (..., name, name, ...) will be used, in order, to supply data
       values for the remaining "no data" cells (or cells with value 0 with -z flag).

       In other words, the first raster map is used first and if it had  some  "no  data"  cells,
       then  second  raster map is used for these cells, then the third and so on.  So the formal
       command line syntax can be also written as input=primary,secondary,tertiary,...   For  two
       raster  maps,  the  first  one can be viewed as the primary one or the default one and the
       second one as the secondary one or a fallback.

       Figure: Result of patching of two raster maps using the -z flag to treat zeros  as  NULLs.
       Note the value 1 being preserved from the first raster while the value 6 is taken from the
       second raster instead of the value 0 from the first raster because zeros are replaced with
       the -z flag active.

   Relation to SQL COALESCE() function
       The module is corresponds to the SQL COALESCE() function.  This function takes two or more
       arguments and returns a copy of its first non-NULL argument. If all  arguments  are  NULL,
       the function returns NULL.

       The r.patch module iterates over all cells and for each cell of the output raster map uses
       the first corresponding non-NULL cell in the series of the input raster maps.

   Example of filling areas
       Below, the raster map layer on the far left is patched with the middle  (patching)  raster
       map  layer,  to  produce the composite raster map layer on the right.  The example assumes
       zero values to be treated as NULLs (-z flag).
         1 1 1 0 2 2 0 0    0 0 1 1 0 0 0 0    1 1 1 1 2 2 0 0
         1 1 0 2 2 2 0 0    0 0 1 1 0 0 0 0    1 1 1 2 2 2 0 0
         3 3 3 3 2 2 0 0    0 0 0 0 0 0 0 0    3 3 3 3 2 2 0 0
         3 3 3 3 0 0 0 0    4 4 4 4 4 4 4 4    3 3 3 3 4 4 4 4
         3 3 3 0 0 0 0 0    4 4 4 4 4 4 4 4    3 3 3 4 4 4 4 4
         0 0 0 0 0 0 0 0    4 4 4 4 4 4 4 4    4 4 4 4 4 4 4 4
       Switching the patched and the patching raster map layers produces the following results:
         0 0 1 1 0 0 0 0    1 1 1 0 2 2 0 0    1 1 1 1 2 2 0 0
         0 0 1 1 0 0 0 0    1 1 0 2 2 2 0 0    1 1 1 1 2 2 0 0
         0 0 0 0 0 0 0 0    3 3 3 3 2 2 0 0    3 3 3 3 2 2 0 0
         4 4 4 4 4 4 4 4    3 3 3 3 0 0 0 0    4 4 4 4 4 4 4 4
         4 4 4 4 4 4 4 4    3 3 3 0 0 0 0 0    4 4 4 4 4 4 4 4
         4 4 4 4 4 4 4 4    0 0 0 0 0 0 0 0    4 4 4 4 4 4 4 4

NOTES

Frequently, this program is used to patch together adjacent map layers which have been
digitized separately. The program v.mkgrid can be used to make adjacent maps align
neatly.

The user should check the current geographic region settings before running r.patch, to
ensure that the region boundaries encompass all of the data desired to be included in the
composite map and to ensure that the region resolution is the resolution of the desired
data. To set the geographic region settings to one or several raster maps, the g.region
program can be used:
g.region raster=map1[,map2[,...]]

Use of r.patch is generally followed by use of the GRASS programs g.remove and g.rename;
g.remove is used to remove the original (un-patched) raster map layers, while g.rename is
used to then assign to the newly-created composite (patched) raster map layer the name of
the original raster map layer.

r.patch reads the existing category label files and color tables from the input maps and
creates these files for the patched, composite output map. This can be quite time
consuming for certain maps, especially if there are many different category values across
the patched maps. The -s flag allows disabling the reading and creation of these support
files, meaning that the output map will have no category labels and no explicit color
table.

Number of raster maps to be processed is given by the limit of the operating system. For
example, both the hard and soft limits are typically 1024. The soft limit can be changed
with e.g. ulimit -n 1500 (UNIX-based operating systems) but not higher than the hard
limit. If it is too low, you can as superuser add an entry in
/etc/security/limits.conf
# <domain> <type> <item> <value>
your_username hard nofile 1500
This would raise the hard limit to 1500 file. Be warned that more files open need more
RAM. See also the Wiki page Hints for large raster data processing.

Operating systems usually limit the length of the command line which limits the number of
input raster maps user can pass to the module using the option input. In that case,
r.series can be used instead of r.patch.

PERFORMANCE
By specifying the number of parallel processes with nprocs option, r.patch can run
significantly faster, see benchmarks below.
Figure: Benchmark on the left shows execution time for different number of cells,
benchmark on the right shows execution time for different memory size for 5000x5000
raster. See benchmark scripts in source code. (Intel Core i9-10940X CPU @ 3.30GHz x 28)

To reduce the memory requirements to minimum, set option memory to zero. To take
advantage of the parallelization, GRASS GIS needs to compiled with OpenMP enabled.

EXAMPLES

   Example with three maps
       The input are three maps called roads, water and forest. Primarily, we  want  to  use  the
       values  from  roads,  then  from water and if no other values are available we want to use
       forest.  First we set the computation region  assuming  that  the  all  three  maps  fully
       overlap  and  have  the  same  resolution  (so  we can safely use the just the one without
       further modifications of the region).  Then we perform the patching.
       g.region raster=roads
       r.patch input=roads,water,forest output=result

   Map mosaic example using Bash syntax
       Create a list of maps matching a pattern, extend the region to include them all, and patch
       them together to create a mosaic. Overlapping maps will be used in the order listed.
       MAPS=`g.list type=raster separator=comma pat="map_*"`
       g.region raster=$MAPS -p
       r.patch input=$MAPS output=maps_mosaic

AUTHORS

       Michael Shapiro, U.S. Army Construction Engineering Research Laboratory
       Huidae Cho (-z flag and performance improvement)
       Aaron Saw Min Sern (OpenMP support).

SOURCE CODE

       Available at: r.patch source code (history)

       Accessed: Tuesday Jun 27 11:13:02 2023

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       © 2003-2023 GRASS Development Team, GRASS GIS 8.3.0 Reference Manual