Catchment extraction

Functionality

The Catchment extraction operation constructs catchments; a catchment will be calculated for each stream found in the output map of the Drainage network ordering operation. The operation uses a Flow direction map to determine the flow path of each stream.

As input is required:

the output raster map of the Drainage network ordering operation,
the output raster map of the Flow direction operation.

As output a raster map, a polygon map and an attribute table are produced which all use the ID domain of the input Drainage network ordering map.

The attribute table contains information on each catchment, such as:

area and perimeter of the catchment,
total upstream area, i.e. the area of all catchments that drain into this catchment, etc.

Tip: The obtained catchments can be merged afterwards with the Catchment merge operation.

Example:

Input drainage network ordering map:	Output catchment map (boundaries only), including input drainages:

Stream segments with unique IDs; input streams shown in multiple colors.	For each stream, a catchment is constructed; input streams in blue, output catchment polygon boundaries in green.

Input drainage network ordering map including segment drainage IDs:	Output catchment map (boundaries only) including polygon catchment IDs:

Input stream segments in multiple colors; additional stream label points also in multiple colors.	Output catchment polygon boundaries in green, additional catchment label points in black.

Input map requirements:

The output raster map of a previous Drainage network ordering operation,
The output raster map of a previous Flow direction operation.

Domain of output maps and attribute table:

An output raster map and an output polygon map will be created; these maps will use the ID domain of the input Drainage network ordering map.
An output attribute table will be created, using the same ID domain.
The output raster map and the output polygon map are both linked to the output attribute table.

Columns in the Catchment extraction output attribute table:

domain	The IDs of the table's domain, every record (ID) represents a certain catchment that corresponds to a certain stream.
DrainageID	The ID of the stream that corresponds to this catchment; the ID of the table and this column always show the same number. This column is a value column.
UpstreamLinkCatchment	The IDs of the catchments that directly contribute to the current catchment, e.g. when catchments 5 and 7 flow together into catchment 18, then the UpstreamLinkCatchment column will read for the record with ID 18: {5,7}
DownstreamLinkCatchment	The ID of the catchment into which the current catchment will flow (down-flow). This column is a value column.
Perimeter	The perimeter of the current catchment.
CatchmentArea	The area of the current catchment.
TotalUpstreamArea	The total area of the catchments that directly contribute to the current catchment, i.e. the sum of the areas of the catchments listed in column UpstreamLinkCatchment.
LongestFlowLength	The length of the longest flow path found in this catchment, according to the Flow direction input map.
CenterCatchment	The XY-coordinate at the center of each catchment; these coordinates are shown as points in the fourth picture above. This column is a coordinate column.
CenterDrainage	The XY-coordinate in the middle of each stream segment; these coordinates are shown as points in the third picture above. This column is a coordinate column.

Tip 1:

To obtain label points at the center of the drainages, use the Table to point map operation:

As input table, choose the output table of the Catchment extraction operation,
as single XY column, choose the CenterDrainage column as available in the table,
for the domain of the output point map, you can simply use the table domain.

Tip 2:

To obtain label points at the center of the polygons, use the Table to point map operation:

As input table, choose the output table of the Catchment extraction operation,
as single XY column, choose the CenterCatchment column as available in the table,
for the domain of the output point map, you can simply use the table domain.

Tip 3:

From a label point map, you can create an annotation text layer in a map window.
An annotation text layer may come in handy when you wish to move the position of some labels, or when you wish to produce printed output through a map view.
For more information, refer to ILWIS objects : annotation text or Annotation text editor : functionality.