Flow Length (Spatial Analyst Tools)

Summary

Calculates the upstream or downstream distance, or weighted distance, along the flow path for each cell.

Usage

A primary use of the Flow Length tool is to calculate the length of the longest flow path within a given basin. This measure is often used to calculate the time of concentration of a basin. This would be done using the Upstream option.

The tool can also be used to create distance-area diagrams of hypothetical rainfall and runoff events using the weight raster as an impedance to movement downslope.
The value type for the Flow Length output raster is floating point.
When the output raster format is .crf, this tool supports the Pyramid raster storage environment. Pyramids will be created in the output by default. For any other output format, this environment is not supported, and pyramids will not be created.
See Analysis environments and Spatial Analyst for additional details on the geoprocessing environments that apply to this tool.

Label	Explanation	Data type
Input flow direction raster	The input raster that shows the direction of flow out of each cell. The flow direction raster can be created using the Flow Direction tool.	Raster Layer
Direction of measurement (Optional)	The direction of measurement along the flow path. Downstream—Calculates the downslope distance along the flow path, from each cell to a sink or outlet on the edge of the raster. Upstream—Calculates the longest upslope distance along the flow path, from each cell to the top of the drainage divide.	String
Input weight raster (Optional)	An input raster for applying a weight to each cell. If no weight raster is specified, a default weight of 1 will be applied to each cell. For each cell in the output raster, the result will be the number of cells that flow into it.	Raster Layer

Label

Explanation

Data type

Input flow direction raster

The input raster that shows the direction of flow out of each cell.

The flow direction raster can be created using the Flow Direction tool.

Raster Layer

Direction of measurement

(Optional)

The direction of measurement along the flow path.

Downstream—Calculates the downslope distance along the flow path, from each cell to a sink or outlet on the edge of the raster.
Upstream—Calculates the longest upslope distance along the flow path, from each cell to the top of the drainage divide.

String

Input weight raster

(Optional)

An input raster for applying a weight to each cell.

If no weight raster is specified, a default weight of 1 will be applied to each cell. For each cell in the output raster, the result will be the number of cells that flow into it.

Raster Layer

Return value

Label	Explanation	Data type
Output raster	The output raster that shows for each cell the upstream or downstream distance along a flow path.	Raster

FlowLength(in_flow_direction_raster, {direction_measurement}, {in_weight_raster})

Name Explanation Data type

Name	Explanation	Data type
in_flow_direction_raster	The input raster that shows the direction of flow out of each cell. The flow direction raster can be created using the Flow Direction tool.	Raster Layer
direction_measurement (Optional)	The direction of measurement along the flow path. `DOWNSTREAM`—Calculates the downslope distance along the flow path, from each cell to a sink or outlet on the edge of the raster. `UPSTREAM`—Calculates the longest upslope distance along the flow path, from each cell to the top of the drainage divide.	String
in_weight_raster (Optional)	An input raster for applying a weight to each cell. If no weight raster is specified, a default weight of 1 will be applied to each cell. For each cell in the output raster, the result will be the number of cells that flow into it.	Raster Layer

in_flow_direction_raster

The input raster that shows the direction of flow out of each cell.

The flow direction raster can be created using the Flow Direction tool.

Raster Layer

direction_measurement

(Optional)

The direction of measurement along the flow path.

DOWNSTREAM—Calculates the downslope distance along the flow path, from each cell to a sink or outlet on the edge of the raster.
UPSTREAM—Calculates the longest upslope distance along the flow path, from each cell to the top of the drainage divide.

String

in_weight_raster

(Optional)

An input raster for applying a weight to each cell.

If no weight raster is specified, a default weight of 1 will be applied to each cell. For each cell in the output raster, the result will be the number of cells that flow into it.

Raster Layer

Return value

Name	Explanation	Data type
out_raster	The output raster that shows for each cell the upstream or downstream distance along a flow path.	Raster

Code sample

FlowLength example 1 (Python window)

This example Calculates the downstream distance along the flow path for each cell.

import arcpy
from arcpy import env
from arcpy.sa import *
env.workspace = "C:/sapyexamples/data"
outFlowLength = FlowLength("flowdir", "DOWNSTREAM", "")
outFlowLength.save("c:/sapyexamples/output/outflowlen01")

FlowLength example 2 (stand-alone script)

This example Calculates the downstream distance along the flow path for each cell.

# Name: _Ex_02.py
# Description:
# Requirements: Spatial Analyst extension

# Import system modules
import arcpy
from arcpy import env
from arcpy.sa import *

# Set environment settings
env.workspace = "C:/sapyexamples/data"

# Set local variables
inFlowDirectionRaster = "flowdir"
inWeightRaster = ""
directionType = "DOWNSTREAM"

# Run
outFlowLength = FlowLength(inFlowDirectionRaster, directionType, inWeightRaster)

# Save the output
outFlowLength.save("c:/sapyexamples/output/outflowlen02.tif")

Environments

Auto Commit, Cell Size, Cell Size Projection Method, Current Workspace, Extent, Geographic Transformations, Output CONFIG Keyword, Output Coordinate System, Pyramid, Scratch Workspace, Snap Raster, Tile Size

Licensing information

Basic: Requires Spatial Analyst
Standard: Requires Spatial Analyst
Advanced: Requires Spatial Analyst

Flow Length (Spatial Analyst Tools)

Summary

Usage

Parameters

Return value

Environments

Licensing information

Related topics