Provided by: grass-doc_7.8.2-1build3_all 
NAME
g.region - Manages the boundary definitions for the geographic region.
KEYWORDS
general, settings, computational region, extent, resolution, level1
SYNOPSIS
g.region
g.region --help
g.region [-dsplectwmn3bgfau] [region=name] [raster=name[,name,...]] [raster_3d=name]
[vector=name[,name,...]] [n=value] [s=value] [e=value] [w=value] [t=value] [b=value]
[rows=value] [cols=value] [res=value] [res3=value] [nsres=value] [ewres=value] [tbres=value]
[zoom=name] [align=name] [grow=value] [save=name] [--overwrite] [--help] [--verbose] [--quiet]
[--ui]
Flags:
-d
Set from default region
-s
Save as default region
Only possible from the PERMANENT mapset
-p
Print the current region
-l
Print the current region in lat/long using the current ellipsoid/datum
-e
Print the current region extent
-c
Print the current region map center coordinates
-t
Print the current region in GMT style
-w
Print the current region in WMS style
-m
Print region resolution in meters (geodesic)
-n
Print the convergence angle (degrees CCW)
The difference between the projection’s grid north and true north, measured at the center coordinates
of the current region.
-3
Print also 3D settings
-b
Print the maximum bounding box in lat/long on WGS84
-g
Print in shell script style
-f
Print in shell script style, but in one line (flat)
-a
Align region to resolution (default = align to bounds, works only for 2D resolution)
-u
Do not update the current region
--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:
region=name
Set current region from named region
raster=name[,name,...]
Set region to match raster map(s)
raster_3d=name
Set region to match 3D raster map(s) (both 2D and 3D values)
vector=name[,name,...]
Set region to match vector map(s)
n=value
Value for the northern edge
s=value
Value for the southern edge
e=value
Value for the eastern edge
w=value
Value for the western edge
t=value
Value for the top edge
b=value
Value for the bottom edge
rows=value
Number of rows in the new region
cols=value
Number of columns in the new region
res=value
2D grid resolution (north-south and east-west)
res3=value
3D grid resolution (north-south, east-west and top-bottom)
nsres=value
North-south 2D grid resolution
ewres=value
East-west 2D grid resolution
tbres=value
Top-bottom 3D grid resolution
zoom=name
Shrink region until it meets non-NULL data from this raster map
align=name
Adjust region cells to cleanly align with this raster map
grow=value
Number of cells to add to each side of the current region extent
A negative number shrinks the current region extent
save=name
Save current region settings in named region file
DESCRIPTION
The g.region module allows the user to manage the settings of the current geographic region. These
regional boundaries can be set by the user directly and/or set from a region definition file (stored
under the windows directory in the user’s current mapset). The user can create, modify, and store as
many geographic region definitions as desired for any given mapset. However, only one of these
geographic region definitions will be current at any given moment, for a specified mapset; i.e., GRASS
programs that respect the geographic region settings will use the current geographic region settings.
DEFINITIONS
Region:
In GRASS, a region refers to a geographic area with some defined boundaries, based on a specific map
coordinate system and map projection. Each region also has associated with it the specific east-west
and north-south resolutions of its smallest units (rectangular units called "cells").
The region’s boundaries are given as the northernmost, southernmost, easternmost, and westernmost points
that define its extent (cell edges). The north and south boundaries are commonly called northings, while
the east and west boundaries are called eastings.
The region’s cell resolution defines the size of the smallest piece of data recognized (imported,
analyzed, displayed, stored, etc.) by GRASS modules affected by the current region settings. The
north-south and east-west cell resolutions need not be the same, thus allowing non-square data cells to
exist.
Typically all raster and display modules are affected by the current region settings, but not vector
modules. Some special modules diverge from this rule, for example raster import modules and v.in.region.
Default Region:
Each GRASS LOCATION has a fixed geographic region, called the default geographic region (stored in
the region file DEFAULT_WIND under the special mapset PERMANENT), that defines the extent of the data
base. While this provides a starting point for defining new geographic regions, user-defined
geographic regions need not fall within this geographic region. The current region can be reset to
the default region with the -d flag. The default region is initially set when the location is first
created and can be reset using the -s flag.
Current Region:
Each mapset has a current geographic region. This region defines the geographic area in which all
GRASS displays and raster analyses will be done. Raster data will be resampled, if necessary, to meet
the cell resolutions of the current geographic region setting.
Saved Regions:
Each GRASS MAPSET may contain any number of pre-defined, and named, geographic regions. These region
definitions are stored in the user’s current mapset location under the windows directory (also
referred to as the user’s saved region definitions). Any of these pre-defined geographic regions may
be selected, by name, to become the current geographic region. Users may also access saved region
definitions stored under other mapsets in the current location, if these mapsets are included in the
user’s mapset search path or the ’@’ operator is used (region_name@mapset).
NOTES
After all updates have been applied, the current region’s southern and western boundaries are (silently)
adjusted so that the north/south distance is a multiple of the north/south resolution and that the
east/west distance is a multiple of the east/west resolution.
With the -a flag all four boundaries are adjusted to be even multiples of the resolution, aligning the
region to the resolution supplied by the user. The default is to align the region resolution to match the
region boundaries.
The -m flag will report the region resolution in meters. The resolution is calculated by averaging the
resolution at the region boundaries. This resolution is calculated by dividing the geodesic distance in
meters at the boundary by the number of rows or columns. For example the east / west resolution (ewres)
is determined from an average of the geodesic distances at the North and South boundaries divided by the
number of columns.
The -p (or -g) option is recognized last. This means that all changes are applied to the region settings
before printing occurs.
The -g flag prints the current region settings in shell script style. This format can be given back to
g.region on its command line. This may also be used to save region settings as shell environment
variables with the UNIX eval command, "eval `g.region -g`".
Additional parameter information:
zoom=name
Shrink current region settings to the smallest region encompassing all non-NULL data in the named
raster map layer that fall inside the user’s current region. In this way you can tightly zoom in on
isolated clumps within a bigger map.
If the user also includes the raster=name option on the command line, zoom=name will set the current
region settings to the smallest region encompassing all non-NULL data in the named zoom map that fall
inside the region stated in the cell header for the named raster map.
align=name
Set the current resolution equal to that of the named raster map, and align the current region to a
row and column edge in the named map. Alignment only moves the existing region edges outward to the
edges of the next nearest cell in the named raster map - not to the named map’s edges. To perform
the latter function, use the raster=name option.
EXAMPLES
Printing extent and raster resolution in 2D and 3D
g.region -p
This will print the current region in the format:
projection: 1 (UTM)
zone: 13
datum: nad27
ellipsoid: clark66
north: 4928000
south: 4914000
west: 590000
east: 609000
nsres: 20
ewres: 20
rows: 700
cols: 950
g.region -p3
This will print the current region and the 3D region (used for voxels) in the format:
projection: 1 (UTM)
zone: 13
datum: nad27
ellipsoid: clark66
north: 4928000
south: 4914000
west: 590000
east: 609000
top: 1.00000000
bottom: 0.00000000
nsres: 20
nsres3: 20
ewres: 20
ewres3: 20
tbres: 1
rows: 700
rows3: 700
cols: 950
cols3: 950
depths: 1
g.region -g
The -g option prints the region in the following script style (key=value) format:
n=4928000
s=4914000
w=590000
e=609000
nsres=20
ewres=20
rows=700
cols=950
g.region -bg
The -bg option prints the region in the following script style (key=value) format plus the boundary
box in latitude-longitude/WGS84:
n=4928000
s=4914000
w=590000
e=609000
nsres=20
ewres=20
rows=700
cols=950
LL_W=-103.87080682
LL_E=-103.62942884
LL_N=44.50164277
LL_S=44.37302019
g.region -l
The -l option prints the region in the following format:
long: -103.86789484 lat: 44.50165890 (north/west corner)
long: -103.62895703 lat: 44.49904013 (north/east corner)
long: -103.63190061 lat: 44.37303558 (south/east corner)
long: -103.87032572 lat: 44.37564292 (south/west corner)
rows: 700
cols: 950
Center longitude: 103:44:59.170374W [-103.74977]
Center latitude: 44:26:14.439781N [44.43734]
g.region -pm
This will print the current region in the format (latitude-longitude location):
projection: 3 (Latitude-Longitude)
zone: 0
ellipsoid: wgs84
north: 90N
south: 40N
west: 20W
east: 20E
nsres: 928.73944902
ewres: 352.74269109
rows: 6000
cols: 4800
Note that the resolution is here reported in meters, not decimal degrees.
Changing extent and raster resolution using values
g.region n=7360100 e=699000
will reset the northing and easting for the current region, but leave the south edge, west edge, and
the region cell resolutions unchanged.
g.region n=51:36:05N e=10:10:05E s=51:29:55N w=9:59:55E res=0:00:01
will reset the northing, easting, southing, westing and resolution for the current region, here in
DMS latitude-longitude style (decimal degrees and degrees with decimal minutes can also be used).
g.region -dp s=698000
will set the current region from the default region for the GRASS data base location, reset the south
edge to 698000, and then print the result.
g.region n=n+1000 w=w-500
The n=value may also be specified as a function of its current value: n=n+value increases the
current northing, while n=n-value decreases it. This is also true for s=value, e=value, and w=value.
In this example the current region’s northern boundary is extended by 1000 units and the current
region’s western boundary is decreased by 500 units.
g.region n=s+1000 e=w+1000
This form allows the user to set the region boundary values relative to one another. Here, the
northern boundary coordinate is set equal to 1000 units larger than the southern boundary’s
coordinate value, and the eastern boundary’s coordinate value is set equal to 1000 units larger than
the western boundary’s coordinate value. The corresponding forms s=n-value and
w=e-value may be used to set the values of the region’s southern and western boundaries, relative to the
northern and eastern boundary values.
Changing extent and raster resolution using maps
g.region raster=soils
This form will make the current region settings exactly the same as those given in the cell header
file for the raster map layer soils.
g.region raster=soils zoom=soils
This form will first look up the cell header file for the raster map layer soils, use this as the
current region setting, and then shrink the region down to the smallest region which still
encompasses all non-NULL data in the map layer soils. Note that if the parameter raster=soils were
not specified, the zoom would shrink to encompass all non-NULL data values in the soils map that were
located within the current region settings.
g.region -up raster=soils
The -u option suppresses the re-setting of the current region definition. This can be useful when it
is desired to only extract region information. In this case, the cell header file for the soils map
layer is printed without changing the current region settings.
g.region -up zoom=soils save=soils
This will zoom into the smallest region which encompasses all non-NULL soils data values, and save
the new region settings in a file to be called soils and stored under the windows directory in the
user’s current mapset. The current region settings are not changed.
Changing extent and raster resolution in 3D
g.region b=0 t=3000 tbres=200 res3=100 g.region -p3
This will define the 3D region for voxel computations. In this example a volume with bottom (0m) to
top (3000m) at horizontal resolution (100m) and vertical resolution (200m) is defined.
Using g.region in a shell in combination with OGR
Extracting a spatial subset of the external vector map soils.shp into new external vector map
soils_cut.shp using the OGR ogr2ogr tool:
eval `g.region -g`
ogr2ogr -spat $w $s $e $n soils_cut.shp soils.shp
This requires that the location/SHAPE file projection match.
Using g.region in a shell in combination with GDAL
Extracting a spatial subset of the external raster map p016r035_7t20020524_z17_nn30.tif into new external
raster map p016r035_7t20020524_nc_spm_wake_nn30.tif using the GDAL gdalwarp tool:
eval `g.region -g`
gdalwarp -t_srs "`g.proj -wf`" -te $w $s $e $n \
p016r035_7t20020524_z17_nn30.tif \
p016r035_7t20020524_nc_spm_wake_nn30.tif
Here the input raster map does not have to match the location projection since it is reprojected on the
fly.
SEE ALSO
g.access, g.mapsets, g.proj
Environment variables: GRASS_REGION and WIND_OVERRIDE
AUTHOR
Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
SOURCE CODE
Available at: g.region source code (history)
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© 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual