Computational region - Revision history

Lrntct at 19:37, 2 June 2025

2025-06-02T19:37:03Z

← Older revision		Revision as of 19:37, 2 June 2025
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	##Raster '''output''' maps have their bounds and resolution equal to those of the current computational region.		##Raster '''output''' maps have their bounds and resolution equal to those of the current computational region.
	##Vector maps are always '''considered completely'''.		##Vector maps are always '''considered completely'''.

			Note that in GRASS, 2D and 3D raster maps share the same region extent. However, the 3D resolution is distinct from the 2D resolution, and they each must be set separately.

	'''Hint''' for users migrating from other software packages:<br>		'''Hint''' for users migrating from other software packages:<br>

⚠️Mlennert: /* Absolute values data */

2016-11-29T00:38:21Z

Absolute values data

← Older revision		Revision as of 00:38, 29 November 2016
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	[[Image:Grid_examples.png\|350px\|center\|thumb\|Grid representation of the two raster cell settings: 9x9 and 10x10 within the same spatial extent]]		[[Image:Grid_examples.png\|350px\|center\|thumb\|Grid representation of the two raster cell settings: 9x9 and 10x10 within the same spatial extent]]

	The automatic, on-the-fly nearest neighbor resampling will thus pick the 81 cells in the raster map whose centers are closest to the centers of the cells in the computational regions and affect the values of these cells to the resampling output. In other words, the values of 19 cells are "lost". When you now calculate the sum of the values with {{cmd\|r.univar}} the result will be different than if you adjust the ~~computation~~ region to the original map:		The automatic, on-the-fly nearest neighbor resampling will thus pick the 81 cells in the raster map whose centers are closest to the centers of the cells in the computational regions and affect the values of these cells to the resampling output. In other words, the values of 19 cells are "lost". When you now calculate the sum of the values with {{cmd\|r.univar}} the result will be different than if you adjust the computational region to the original map:

	<source lang="bash">		<source lang="bash">

⚠️Mlennert: /* Absolute values data */

2016-11-29T00:37:00Z

Absolute values data

← Older revision		Revision as of 00:37, 29 November 2016
Line 33:		Line 33:
	[[Image:Grid_examples.png\|350px\|center\|thumb\|Grid representation of the two raster cell settings: 9x9 and 10x10 within the same spatial extent]]		[[Image:Grid_examples.png\|350px\|center\|thumb\|Grid representation of the two raster cell settings: 9x9 and 10x10 within the same spatial extent]]

	The automatic, on-the-fly nearest neighbor resampling will thus pick the 81 cells in the raster map whose centers are closest to the centers of the cells in the computational regions and affect the values of these cells to the resampling output. In other words, the values of 19 cells are "lost". When you now calculate the sum of the values with {{cmd\|r.univar} the result will be different than if you adjust the computation region to the original map:		The automatic, on-the-fly nearest neighbor resampling will thus pick the 81 cells in the raster map whose centers are closest to the centers of the cells in the computational regions and affect the values of these cells to the resampling output. In other words, the values of 19 cells are "lost". When you now calculate the sum of the values with {{cmd\|r.univar}} the result will be different than if you adjust the computation region to the original map:

	<source lang="bash">		<source lang="bash">

⚠️Mlennert: /* Understanding the impact of region settings */

2016-11-28T14:20:51Z

Understanding the impact of region settings

@@ Line 19: / Line 19: @@
 If a raster map has an original resolution that is different from the resolution of the current computational region, but also when the current computational region's cells are shifted in space compared to the original raster map, the values are resampled on-the-fly by GRASS GIS, using the nearest neighbor method (see some of the details in the [https://grass.osgeo.org/grass70/manuals/rasterintro.html#raster-map-operations Raster introduction manual].
-As an example, imagine a simple maps which is 10 rows and 10 columns. Imagine also a computational region with the same extent but of 9 rows and 9 columns. Cells in the computational region are thus slightly bigger than cells in the original raster map, and there are 100 cells in the map, but only 81 cells in the region.
+The nature of the impact depends on the nature of the data. If the data represents a continuous field, but not absolute values that depends on the size of the space, then nearest neighbor resampling might slightly alter your data, but not fundamentally. Examples of continuous field data are elevation, average rainfall, concentration of pollutants in soil, water or air, land surface reflectance (satellite imagery), etc. Examples of absolute values are number of inhabitants, total rainfall per pixel, total turnover of enterprises, etc.
 [[Image:Grid_examples.png|350px|center|thumb|Grid representation of the two raster cell settings: 9x9 and 10x10 within the same spatial extent]]
@@ Line 26: / Line 36: @@
 <source lang="bash">
-g.region s=0 n=10 w=0 e=10 res=1
+g.region s=0 n=10 w=0 e=10 res=1 -p
 r.mapcalc "tenbyten = 1"
 r.univar tenbyten
 [...]
 sum: 100
-g.region cols=9 rows=9
+g.region cols=9 rows=9 -p
 r.univar tenbyten
 [...]

⚠️Mlennert: added an image to visualise different raster grids

2016-11-28T08:23:03Z

added an image to visualise different raster grids

← Older revision		Revision as of 08:23, 28 November 2016
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	If a raster map has an original resolution that is different from the resolution of the current computational region, but also when the current computational region's cells are shifted in space compared to the original raster map, the values are resampled on-the-fly by GRASS GIS, using the nearest neighbor method (see some of the details in the [https://grass.osgeo.org/grass70/manuals/rasterintro.html#raster-map-operations Raster introduction manual].		If a raster map has an original resolution that is different from the resolution of the current computational region, but also when the current computational region's cells are shifted in space compared to the original raster map, the values are resampled on-the-fly by GRASS GIS, using the nearest neighbor method (see some of the details in the [https://grass.osgeo.org/grass70/manuals/rasterintro.html#raster-map-operations Raster introduction manual].

	As an example, imagine a simple maps which is 10 rows and 10 columns. Imagine also a computational region with the same extent but of 9 rows and 9 columns. Cells in the computational region are thus slightly bigger than cells in the original raster map, and there are 100 cells in the map, but only 81 cells in the region. The automatic, on-the-fly nearest neighbor resampling will thus pick the 81 cells in the raster map whose centers are closest to the centers of the cells in the computational regions and affect the values of these cells to the resampling output. In other words, the values of 19 cells are "lost". When you now calculate the sum of the values with {{cmd\|r.univar} the result will be different than if you adjust the computation region to the original map:		As an example, imagine a simple maps which is 10 rows and 10 columns. Imagine also a computational region with the same extent but of 9 rows and 9 columns. Cells in the computational region are thus slightly bigger than cells in the original raster map, and there are 100 cells in the map, but only 81 cells in the region.

			[[Image:Grid_examples.png\|350px\|center\|thumb\|Grid representation of the two raster cell settings: 9x9 and 10x10 within the same spatial extent]]

			The automatic, on-the-fly nearest neighbor resampling will thus pick the 81 cells in the raster map whose centers are closest to the centers of the cells in the computational regions and affect the values of these cells to the resampling output. In other words, the values of 19 cells are "lost". When you now calculate the sum of the values with {{cmd\|r.univar} the result will be different than if you adjust the computation region to the original map:

	<source lang="bash">		<source lang="bash">

⚠️Mlennert: Added an explanation of the impacts of the automatic nearest neighbor resampling on raster analyses.

2016-11-28T08:14:49Z

Added an explanation of the impacts of the automatic nearest neighbor resampling on raster analyses.

@@ Line 13: / Line 13: @@
 The current region or computational region can be considered as ''region of interest'' (ROI).
-=== FAQs  ===
+== Understanding the impact of region settings  ==
 '''Q:''' I don't see anything!

Neteler: +hint about ROI

2013-12-30T15:31:37Z

+hint about ROI

← Older revision		Revision as of 15:31, 30 December 2013
Line 9:		Line 9:
	##Raster '''output''' maps have their bounds and resolution equal to those of the current computational region.		##Raster '''output''' maps have their bounds and resolution equal to those of the current computational region.
	##Vector maps are always '''considered completely'''.		##Vector maps are always '''considered completely'''.

			'''Hint''' for users migrating from other software packages:<br>
			The current region or computational region can be considered as ''region of interest'' (ROI).

	=== FAQs ===		=== FAQs ===

Neteler: +See also Location and Mapsets

2013-12-22T12:23:05Z

+See also Location and Mapsets

← Older revision		Revision as of 12:23, 22 December 2013
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	r.mapcalc "subset = original_map"		r.mapcalc "subset = original_map"
	</source>		</source>

			=== See also ===

			* [[Location and Mapsets]]

	[[Category: Documentation]]		[[Category: Documentation]]
	[[Category: FAQ]]		[[Category: FAQ]]
	[[Category: Tutorial]]		[[Category: Tutorial]]

Neteler: +How to subset a raster map

2013-09-07T11:35:33Z

+How to subset a raster map

← Older revision		Revision as of 11:35, 7 September 2013
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	'''A:''' Typically the computational region is set to an area not covering the raster map of interest. Use {{cmd\|g.region}} to adjust the computational region settings, e.g. by setting it to the raster map:		'''A:''' Typically the computational region is set to an area not covering the raster map of interest. Use {{cmd\|g.region}} to adjust the computational region settings, e.g. by setting it to the raster map:
			<source lang="bash">
	g.region rast=myrastermap -p		g.region rast=myrastermap -p
			</source>

	'''Using the graphical user interface:'''		'''Using the graphical user interface:'''
Line 31:		Line 32:

	'''A:''' Sometimes, the resolution of the computational region is not matching exactly the resolution entered to {{cmd\|g.region}}. Here's an example:		'''A:''' Sometimes, the resolution of the computational region is not matching exactly the resolution entered to {{cmd\|g.region}}. Here's an example:
	g.region rast=myrastermap res=1 -p		<source lang="bash">
	(...)		g.region rast=myrastermap res=1 -p
	nsres: 0.9993515		(...)
	ewres: 1.00025576		nsres: 0.9993515
	(...)		ewres: 1.00025576
			(...)
			</source>

	To force the computational region to match the resolution entered, you need to use the -a flag:		To force the computational region to match the resolution entered, you need to use the -a flag:
	g.region rast=myrastermap res=1 -ap		<source lang="bash">
	(...)		g.region rast=myrastermap res=1 -ap
	nsres: 1		(...)
	ewres: 1		nsres: 1
	(...)		ewres: 1
			(...)
			</source>

	<br> '''Q:''' The raster map looks ugly.		<br> '''Q:''' The raster map looks ugly.
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	'''A:''' You likely try to use more memory than your computer offers which is commonly caused by a too high raster resolution (or too large computational region). E.g., it is pointless to calculate common DEM data at nanometer raster resolution. Set the region extent and raster resolution properly with {{cmd\|g.region}} (in the wxGUI: menu Settings -> Region -> Display Region \| Set Region).		'''A:''' You likely try to use more memory than your computer offers which is commonly caused by a too high raster resolution (or too large computational region). E.g., it is pointless to calculate common DEM data at nanometer raster resolution. Set the region extent and raster resolution properly with {{cmd\|g.region}} (in the wxGUI: menu Settings -> Region -> Display Region \| Set Region).

			'''Q:''' I want to subset a raster map!

			'''A:''' While this is done on the fly by setting the computational region properly, you can still create a subset raster map copy with
			<source lang="bash">
			r.mapcalc "subset = original_map"
			</source>

	[[Category: Documentation]]		[[Category: Documentation]]
	[[Category: FAQ]]		[[Category: FAQ]]
	[[Category: Tutorial]]		[[Category: Tutorial]]

⚠️PierreRoudier: Added FAQ entry on the -a flag

2012-04-29T23:56:28Z

Added FAQ entry on the -a flag

← Older revision		Revision as of 23:56, 29 April 2012
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	[[Image:Wxgui computat region to raster.png\|350px\|center\|thumb\|wxGUI: set computational region to selected map (right mouse button context menu on map name)]]		[[Image:Wxgui computat region to raster.png\|350px\|center\|thumb\|wxGUI: set computational region to selected map (right mouse button context menu on map name)]]

			<br> '''Q:''' The resolution of my region is not the one I asked for!

			'''A:''' Sometimes, the resolution of the computational region is not matching exactly the resolution entered to {{cmd\|g.region}}. Here's an example:
			g.region rast=myrastermap res=1 -p
			(...)
			nsres: 0.9993515
			ewres: 1.00025576
			(...)

			To force the computational region to match the resolution entered, you need to use the -a flag:
			g.region rast=myrastermap res=1 -ap
			(...)
			nsres: 1
			ewres: 1
			(...)



	<br> '''Q:''' The raster map looks ugly.		<br> '''Q:''' The raster map looks ugly.