Watershed algorithm - Revision history

Rudolf@tygron.nl at 11:38, 17 October 2025

2025-10-17T11:38:40Z

← Older revision		Revision as of 11:38, 17 October 2025
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	In the study of image processing, a watershed<ref~~>https://en.wikipedia.org/~~wiki~~/Watershed_%28image_processing%29</ref>~~ is a transformation defined on a grayscale image. Some of the proposed algorithms use the principle of flooding water on a DEM to identify individual watersheds, separated by borders of higher altitude.		In the study of image processing, a watershed{{ref\|wiki}} is a transformation defined on a grayscale image. Some of the proposed algorithms use the principle of flooding water on a DEM to identify individual watersheds, separated by borders of higher altitude.

	One of the problems that often occur when generated watersheds is over-segmentation. Over-segmentation occurs when the elevation data contains many local minima. To overcome this issue, the elevation data is often smoothed to remove local minima and generate larger watersheds.		One of the problems that often occur when generated watersheds is over-segmentation. Over-segmentation occurs when the elevation data contains many local minima. To overcome this issue, the elevation data is often smoothed to remove local minima and generate larger watersheds.
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	Our watershed algorithm is used for a slightly different purpose, namely identifying the watershed areas for predefined waterways. More generally, one is interested into which waterway a set of rain drops will end up based on the terrain elevation and the starting location of the drops.		Our watershed algorithm is used for a slightly different purpose, namely identifying the watershed areas for predefined waterways. More generally, one is interested into which waterway a set of rain drops will end up based on the terrain elevation and the starting location of the drops.

	Since we support the calculation of rainfall using our [[~~Water_Module\|water module~~]] and can generate water flow direction results, we have altered the watershed algorithm to use the flow direction instead of the terrain elevation as input. Given that the rainfall used is adequately large, it helps us reducing the amount of local minima and hopefully generate more realistic results based on rainfall simulation.		Since we support the calculation of rainfall using our [[Water Module]] and can generate water flow direction results, we have altered the watershed algorithm to use the flow direction instead of the terrain elevation as input. Given that the rainfall used is adequately large, it helps us reducing the amount of local minima and hopefully generate more realistic results based on rainfall simulation.

	==Variants==		==Variants==
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	==Data initialization==		==Data initialization==
	The basic watershed algorithm has been adjusted to allow users to mark areas and [[Waterway~~\|waterways~~]] and [[Water ~~bodies\|water~~ bodies]] as unique discharge areas, present at the start of the algorithm.		The basic watershed algorithm has been adjusted to allow users to mark areas and [[Waterway]]s and [[Water bodies]] as unique discharge areas, present at the start of the algorithm.

	Additionally, culverts that connect waterways can be taken into account to automatically expand to free water cells and override discharge area of water cell created for waterways and waterbodies.		Additionally, culverts that connect waterways can be taken into account to automatically expand to free water cells and override discharge area of water cell created for waterways and waterbodies.
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	==Notes==		==Notes==
	* The watershed algorithm implementation is an adapted version of the algorithm proposed by Vitor et. al. <ref~~>Vitor, Giovani & Körbes, André & Lotufo, Roberto & Ferreira, Janito. (2010). Analysis of a Step-Based Watershed Algorithm Using CUDA. IJNCR. 1. 16-28. 10.4018/jncr.2010100102. </ref>~~.		* The watershed algorithm implementation is an adapted version of the algorithm proposed by Vitor et. al.{{ref\|vitor}}.
	* The algorithm uses 3 types of cells:		* The algorithm uses 3 types of cells:
	** discharge area cells, either initial or created by the algorithm;		** discharge area cells, either initial or created by the algorithm;
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	* When using a Water Overlay's Flow direction result as an input for the Watershed Overlay, it is important to consider what rain settings are used. Generally, you want to select a rainfall big enough for local minima due to small depressions in the terrain to disappear. On the other side, the rainfall should not be too big for the marked ditches it should end up in.		* When using a Water Overlay's Flow direction result as an input for the Watershed Overlay, it is important to consider what rain settings are used. Generally, you want to select a rainfall big enough for local minima due to small depressions in the terrain to disappear. On the other side, the rainfall should not be too big for the marked ditches it should end up in.

	~~==See also=~~=		{{article end
			\|seealso=
	* [[Watershed Module Theory]]		* [[Watershed Module Theory]]
			\|references=
	==~~References~~==		<references>
	<references/>		{{ref\|wiki
			\|name=Watershed (Image Processing)
			\|author=
			\|page=
			\|source=
			\|link=https://en.wikipedia.org/wiki/Watershed_%28image_processing%29
			\|lastvisit=2025-10-17
			}}
			{{ref\|vitor
			\|name=Analysis of a Step-Based Watershed Algorithm Using CUDA
			\|author=Vitor, Giovani & Körbes, André & Lotufo, Roberto & Ferreira, Janito. (2010)
			\|page=
			\|source=1. 16-28. 10.4018/jncr.2010100102
			\|link=
			\|lastvisit=
			}}
			</references>
			}}

Frank@tygron.nl at 13:40, 2 February 2024

2024-02-02T13:40:01Z

← Older revision		Revision as of 13:40, 2 February 2024
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	# '''Flood''' ''(loop until stable)'': Cells referencing a cell with a discharge area id are updated to that same id.		# '''Flood''' ''(loop until stable)'': Cells referencing a cell with a discharge area id are updated to that same id.
	# '''Fill''' ''(loop until stable)'': Assign a watershed to cells that neighbor a watershed and that do not yet belong to a watershed. Optionally [[Limit road (Watershed Overlay)\|limit]] this step to cells that are not roads.		# '''Fill''' ''(loop until stable)'': Assign a watershed to cells that neighbor a watershed and that do not yet belong to a watershed. Optionally [[Limit road (Watershed Overlay)\|limit]] this step to cells that are not roads.
	# '''Store [[~~Watershed~~ result type (Watershed Overlay)\|watersheds]] and [[Base types result type (Watershed Overlay)\|base types]] results'''.		# '''Store [[Watersheds result type (Watershed Overlay)\|watersheds]] and [[Base types result type (Watershed Overlay)\|base types]] results'''.

	==Notes==		==Notes==

Frank@tygron.nl at 13:39, 1 March 2023

2023-03-01T13:39:05Z

← Older revision		Revision as of 13:39, 1 March 2023
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	==Variants==		==Variants==
	The Watershed algorithm has two variants, one based on absolute height values of the [[~~Elevation Model\|~~DTM]] and one based on a [[Surface avg direction result type (Water Overlay)\|Water Overlay's Surface Direction]] result type.		The Watershed algorithm has two variants, one based on absolute height values of the [[DTM]] and one based on a [[Surface avg direction result type (Water Overlay)\|Water Overlay's Surface Direction]] result type.

	==Data initialization==		==Data initialization==

Godelief@tygron.com: /* Variants */

2020-09-04T08:44:03Z

Variants

← Older revision		Revision as of 08:44, 4 September 2020
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	==Variants==		==Variants==
	The Watershed algorithm has two variants, one based on absolute height values of the [[~~Digital terrain model~~\|DTM]] and one based on a [[Surface avg direction result type (Water Overlay)\|Water Overlay's Surface Direction]] result type.		The Watershed algorithm has two variants, one based on absolute height values of the [[Elevation Model\|DTM]] and one based on a [[Surface avg direction result type (Water Overlay)\|Water Overlay's Surface Direction]] result type.

	==Data initialization==		==Data initialization==

Rudolf@tygron.nl at 12:30, 14 August 2020

2020-08-14T12:30:08Z

← Older revision		Revision as of 12:30, 14 August 2020
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	==Variants==		==Variants==
	The Watershed algorithm has two variants, one based on absolute height values of the [[Digital terrain model\|DTM]] and one based on a [[Surface ~~average~~ direction result type (Water Overlay)\|Water Overlay's Surface Direction]] result type.		The Watershed algorithm has two variants, one based on absolute height values of the [[Digital terrain model\|DTM]] and one based on a [[Surface avg direction result type (Water Overlay)\|Water Overlay's Surface Direction]] result type.

	==Data initialization==		==Data initialization==

Godelief@tygron.com at 08:03, 12 June 2020

2020-06-12T08:03:34Z

← Older revision		Revision as of 08:03, 12 June 2020
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	Our watershed algorithm is used for a slightly different purpose, namely identifying the watershed areas for predefined waterways. More generally, one is interested into which waterway a set of rain drops will end up based on the terrain elevation and the starting location of the drops.		Our watershed algorithm is used for a slightly different purpose, namely identifying the watershed areas for predefined waterways. More generally, one is interested into which waterway a set of rain drops will end up based on the terrain elevation and the starting location of the drops.

	Since we support the calculation of rainfall using our water module and can generate water flow direction results, we have altered the watershed algorithm to use the flow direction instead of the terrain elevation as input. Given that the rainfall used is adequately large, it helps us reducing the amount of local minima and hopefully generate more realistic results based on rainfall simulation.		Since we support the calculation of rainfall using our [[Water_Module\|water module]] and can generate water flow direction results, we have altered the watershed algorithm to use the flow direction instead of the terrain elevation as input. Given that the rainfall used is adequately large, it helps us reducing the amount of local minima and hopefully generate more realistic results based on rainfall simulation.

	==Variants==		==Variants==

Hedi@tygron.com: /* Tips */

2020-06-11T15:03:08Z

Tips

← Older revision		Revision as of 15:03, 11 June 2020
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	==Tips==		==Tips==
	* When using a Water Overlay's Flow direction result as an input for the Watershed Overlay, it is important to consider what rain settings are used. Generally, you want select a rainfall big enough for local minima, due to small depressions in the terrain, to disappear. On the other side, the rainfall should not be too big for the marked ditches it should end up in.		* When using a Water Overlay's Flow direction result as an input for the Watershed Overlay, it is important to consider what rain settings are used. Generally, you want to select a rainfall big enough for local minima due to small depressions in the terrain to disappear. On the other side, the rainfall should not be too big for the marked ditches it should end up in.

	==See also==		==See also==

Hedi@tygron.com: /* Notes */

2020-06-11T14:39:04Z

Notes

← Older revision		Revision as of 14:39, 11 June 2020
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	*** discharge area cell, which will receive a unique id.		*** discharge area cell, which will receive a unique id.
	* The cells that are updated during the Flood step depend in the direction as well on the selected options. If the creation of additional discharge areas was not allowed, certain sections will not belong to a watershed.		* The cells that are updated during the Flood step depend in the direction as well on the selected options. If the creation of additional discharge areas was not allowed, certain sections will not belong to a watershed.
	* Directions from previous steps are ignored in the Fill step. Instead, ~~cell~~ that do not belong to a watershed are updated to a neighboring watershed, unless the cell is a road and the option [[Limit road (Watershed Overlay)\|Limit road]] is selected. When that setting is active, the watershed expansion in the fill step will not pass roads.		* Directions from previous steps are ignored in the Fill step. Instead, cells that do not belong to a watershed are updated to a neighboring watershed, unless the cell is a road and the option [[Limit road (Watershed Overlay)\|Limit road]] is selected. When that setting is active, the watershed expansion in the fill step will not pass roads.

	==Tips==		==Tips==

Frank@tygron.nl at 16:38, 10 June 2020

2020-06-10T16:38:41Z

← Older revision		Revision as of 16:38, 10 June 2020
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	==Variants==		==Variants==
	The Watershed algorithm has two variants, one based on absolute height values of the [[Digital terrain model\|DTM]] and one based on a [[Surface direction result type (Water Overlay)\|Water Overlay's Surface Direction]] result type.		The Watershed algorithm has two variants, one based on absolute height values of the [[Digital terrain model\|DTM]] and one based on a [[Surface average direction result type (Water Overlay)\|Water Overlay's Surface Direction]] result type.

	==Data initialization==		==Data initialization==

Frank@tygron.nl: /* Watershed Algorithm */

2020-06-10T15:29:03Z

Watershed Algorithm

← Older revision		Revision as of 15:29, 10 June 2020
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	# '''Convert plateaus to directions''' ''(loop until stable)'': Cells without a reference to others are re-evaluated. Such a cell will reference the first neighboring cell that flow away from it.		# '''Convert plateaus to directions''' ''(loop until stable)'': Cells without a reference to others are re-evaluated. Such a cell will reference the first neighboring cell that flow away from it.
	# '''Fix self directions''' ''(only once)'': Neighboring cells that reference each other, are both set back to being a plateau. They can now be identified as a unique discharge area, depending on the selected options.		# '''Fix self directions''' ''(only once)'': Neighboring cells that reference each other, are both set back to being a plateau. They can now be identified as a unique discharge area, depending on the selected options.
	# '''Store ~~directions as a~~ [[Direction result type (Watershed Overlay)\|result type]]'''.		# '''Store [[Direction result type (Watershed Overlay)\|directions]] as a result type'''.
	# '''Add discharge areas for remaining plateaus''' ''(optional, only once)'': For cells that are marked as a plateau, mark them now as a discharge area with a unique id depending on the selected options.		# '''Add discharge areas for remaining plateaus''' ''(optional, only once)'': For cells that are marked as a plateau, mark them now as a discharge area with a unique id depending on the selected options.
	# '''Propagate the created discharge area''' ''(optional, loop until stable)'': In case two neighboring cells are both discharge areas, combine them by selecting the lowest ids of the two.		# '''Propagate the created discharge area''' ''(optional, loop until stable)'': In case two neighboring cells are both discharge areas, combine them by selecting the lowest ids of the two.