g.region(1)

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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© 2003-2021 GRASS Development Team, GRASS GIS 7.8.6 Reference Manual