## DESCRIPTION

The g.region program 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. 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 programs 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.

Default Region:
  Each GRASS `LOCATION_NAME` 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.
Current Region:
  Each mapset has a current geographic region. This region defines the geographic area in which all GRASS displays and analyses will be done. Data will be resampled, if necessary, to meet the cell resolutions of the current geographic region setting.
Region Data Base:
  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 data base of 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.

## REGION EDIT PROMPT

Most of the options will require the user to edit a geographic region, be it the current geographic region or one stored in the user's data base of region definitions (the `windows` directory). A standard prompt is used to perform this edit. An example is shown below:

    ---------------------------------------------------------------
    |                         IDENTIFY REGION                     |
    |                                                             |
    |           ===========  DEFAULT REGION  ==========           |
    |           |    Default North: 3402025.00        |           |
    |           |                                     |           |
    |           |          ===YOUR REGION===          |           |
    |           |          |  NORTH EDGE   |          |           |
    |           |          |  3402025.00_  |          |           |
    |           |          |               |          |           |
    | Def West: |WEST EDGE |               |EAST EDGE | Def.East: |
    | 233975.00 |233975.00_|               |236025.00_| 236025.00 |
    |           |          |  SOUTH EDGE   |          |           |
    |           |          |  3399975.00_  |          |           |
    |           |          =================          |           |
    |           |                                     |           |
    |           |    Default South: 3399975.00        |           |
    |           =======================================           |
    |                                                             |
    |              Default   GRID RESOLUTION   Region             |
    |               50.00   --- East-West ---  50.00__            |
    |               50.00   -- North-South --  50.00__            |
    |                                                             |
    |                                                             |
    |     AFTER COMPLETING ALL ANSWERS, HIT <ESC> TO CONTINUE     |
    ---------------------------------------------------------------

The fields NORTH EDGE, SOUTH EDGE, WEST EDGE and EAST EDGE, are the boundaries of the geographic region that the user can change. The fields Default North, Default South, Def West and Def East are the boundaries of the default geographic region that are displayed for reference and cannot be changed. The two GRID RESOLUTION Region fields (east-west, and north-south) are the geographic region's cell resolutions that the user can change. The two GRID RESOLUTION Default fields list the resolutions of the default geographic region; these are displayed for reference and cannot be changed here by the user.

## REGION MANAGEMENT MENU OPTIONS

1. Modify the current geographic region directly Allows the user to edit the current region.
2. Set current geographic region from default region Copies the default region to the current geographic region, and then lets the user edit the current geographic region.
3. Set current geographic region from a data base geographic region Allows the user to select a geographic region by name from the data base of geographic regions to become the current geographic region, and then lets the user edit the current geographic region.

   Note: geographic region definition files may be selected from other mapsets as well, if accessible and in the user's mapset search path.

4. Set current geographic region from a raster (cell) map layer Allows the user to select a raster map layer, copies the cell header for this map layer to the current geographic region, and then lets the user edit the current geographic region. This option is useful when subsequent GRASS operations will be used to produce a raster map layer from one input raster map layer and it is necessary that the result coincide with the input raster map layer.
5. Save the current geographic region (window) in the data base Allows the user to save the current geographic region settings in the user's data base of such settings. These files are stored in the `windows` directory under the user's current mapset. This option is useful when the current geographic region is set directly using option 2, or even by another GRASS program (e.g., d.m). This option installs an otherwise temporary geographic region setting into the geographic region definition data base for recall when needed.
6. Create a new data base geographic region setting Creates a new geographic region definition in the user's data base of such settings in the `windows` directory under the current mapset, using the geographic region edit prompt described above. After the geographic region definition is created, the user is asked if this geographic region setting should also be used as the current geographic region.
7. Modify a data base geographic region setting Modifies a geographic region setting (in the data base of such settings in the `windows` directory of the current mapset), using the geographic region edit prompt. After the changes have been made, the user is asked if this geographic region setting should also be used as the current geographic region.

## NON-INTERACTIVE PROGRAM USE

Alternately, the user can modify the settings of the current geographic region by specifying all needed parameters on the command line. The user enters the command g.region parms, where parms are the following parameters and/or flags:

## EXAMPLES

    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 (but also decimal degrees can 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.

    g.region rast=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 -up rast=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 -p rast=soils

  This will zoom into the smallest region which encompasses all non-zero 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.

    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.

    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.

The -p (or -g) option is recognized last. This means that all changes are applied to the region settings before printing occurs.

## NOTE

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.

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.

## SEE ALSO

d.zoom
g.access
g.mapsets
g.projinfo

## AUTHOR

Michael Shapiro, U.S.Army Construction Engineering Research Laboratory

Last changed: $Date: 2005/12/22 14:37:05 $