topic Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path in ArcGIS Spatial Analyst Questions

using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

AnneWynn — Thu, 06 Oct 2016 18:29:33 GMT

How do you find the longest flow path with the Flow Length tool in Spatial Analyst's Hydrology Toolbox? In the ArcGIS Help, it states "A primary use of the Flow Length tool is to calculate the length of the longest flow path within a given basin".

How? I know that Arc Hydro has a tool that does this, and I have used it before - but I was wondering if it is possible to find the longest flow path with the output from the Flow Length tool without using the ArcHydro tool.

Thanks for any insight you can provide!

A. Wynn

Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

ChrisDonohue__GISP — Thu, 06 Oct 2016 19:16:29 GMT

Typically, one would run the Flow Length tool on a hydrologically-correct elevation surface, then take the output raster and generate lines from it using Stream to Feature, then use the shape.length property of the lines to check the distance.

ArcGIS Help (10.2, 10.2.1, and 10.2.2) - overview of the Hydrology toolset

ArcGIS Help (10.2, 10.2.1, and 10.2.2) - Flow Length

ArcGIS Help (10.2, 10.2.1, and 10.2.2) - Stream to Feature

ArcGIS Help (10.2, 10.2.1, and 10.2.2) - Flow Direction

Notes:

- These all require access to the Spatial Analyst Extension.

- Before one can run Flow Length and Stream to Feature, one needs to run Flow Direction. It is assumed at this point that one has a "depressionless DEM" already; if not, there are several processes that will need to be run first.

ArcGIS Help (10.2, 10.2.1, and 10.2.2) - Creating a Depressionless DEM

Chris Donohue, GISP

Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

AnneWynn — Thu, 06 Oct 2016 20:00:27 GMT

Hi Chris,

I had initially thought of that, but my watershed area 55 sq. miles. There are 50519 stream segments in the attribute table. Should I parse it out by grid_code value first?

Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

ChrisDonohue__GISP — Thu, 06 Oct 2016 21:19:48 GMT

Hmmm, I haven't done one of these in a few years, so I'm scratching my head trying to remember what I did to find the longest length once I had the stream segments. I remember adding a Stream Order and a Stream Link to help organize the output, but forget what the process was to find the longest combination from the outputs.

ArcGIS Help (10.2, 10.2.1, and 10.2.2) How Stream Order Works

ArcGIS Help (10.2, 10.2.1, and 10.2.2) Stream Order

ArcGIS Help (10.2, 10.2.1, and 10.2.2) Stream Link

Chris Donohue, GISP

Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

ChrisDonohue__GISP — Thu, 06 Oct 2016 22:26:24 GMT

An alternative method to find the longest flow length uses a Geometric Network after deriving the stream network from Spatial Analyst. Refer to this pdf:

Exercise 4. Watershed and Stream Network Delineation

GIS and Water Resources, Fall 2013

Prepared by David G Tarboton and David R. Maidment

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0ahUKEwjPoomymsfPAhVP2mMKHa3xD4oQFggsMAI&url=http%3A…

See page 57, and also the proceeding section on setting up the Geometric Network. They do state that it is somewhat trial and error, though.

Chris Donohue, GISP

Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

AnneWynn — Fri, 07 Oct 2016 02:18:10 GMT

After running the Flow Length tool, I ran the Stream Order tool, and then used the output from the Stream Order tool for the Stream to Feature tool.

Should I have, perhaps, run the Stream Link tool, then the Stream Order tool, and then the Stream to Feature tool? Would that help structure the stream data further and allow me to find the longest flow path?..

Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

AnneWynn — Fri, 07 Oct 2016 02:19:40 GMT

Thanks for providing this other option, Chris.

I had thought about using a geometric network, but was having difficulties in setting one up.

Best,

Anne

Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

LukeSturtevant — Sun, 12 Dec 2021 03:35:04 GMT

Here is how I've used the spatial analyst tools to return a longest flow path line from the top of a watershed hope this helps:

import arcpy
from arcpy.sa import *
from arcpy import env

watershed =  # Path to watershed polygon
dem =  # Path to DEM
fdr =  # path to flow direction
env.workspace = # path to output workspace
try:
   intermediate = []  # Empty container for intermediate data  
   fdrClip = ExtractByMask(fdr, watershed) # Clip the flow direction raster to the watershed polygon
   arcpy.SetProgressorLabel("Computing longest flow path for the basin...")
   flowLength = Int(FlowLength(fdrClip, "DOWNSTREAM")) # Compute a downstream flow length raster
   arcpy.CalculateStatistics_management(flowLength)
   maxValue = int(arcpy.GetRasterProperties_management(flowLength, "MAXIMUM").getOutput(0)) # Get the maximum cell value from the flow length
   maxCell = Con(flowLength, "1", where_clause="Value = {}".format(repr(maxValue))) # Return the maximum cell from flow length

   lfpRas = CostPath(maxCell, dem, fdrClip, destination_field="Value") # Use cost path to compute raster line from the max cell
   intermediate.append(lfpRas)
   rasLine = arcpy.RasterToPolyline_conversion(lfpRas, "rasLine", "ZERO", simplify="NO_SIMPLIFY") # Generate a polyline from the cost path raster
   intermediate.append(rasLine)
   lfp = arcpy.Dissolve_management(rasLine, "LongestFlowPath") # Dissolve the polyline to generate a single longest flow path line
   intermediate.append(lfp)
finally:
    # Clean up intermediate data
    arcpy.SetProgressorLabel("Removing Intermediate data... ")
    for data in intermediate:
        arcpy.Delete_management(data)
    del intermediate

Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

AnneWynn — Tue, 07 Feb 2017 16:29:29 GMT

Thank you for providing this info, Luke!

Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

ThomasDilts — Sun, 03 Feb 2019 19:10:12 GMT

Hi,

I know that Luke and others have answered this question, but I also wanted to throw out there that I build a toolbox that does this and wrote a blog post on it. The tool is at:

https://www.arcgis.com/home/item.html?id=fc7c1ca07e4844d7b722049b503b0fb2

and the blog post is:

GIS Tools and Techniques for Landscape Ecological Research: Miscellaneous Hydrology Tools for ArcGIS - Find Longest Stre…

The blog post is part of a series that describes the different tools in the Miscellaneous Hydrology Toolbox.

-Tom Dilts

Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

LukeSturtevant — Mon, 04 Feb 2019 15:38:21 GMT

This looks great Thomas thanks. I'm not sure if the download link is working for other people, but it is downloading an empty zip file for me for some reason.

Re: using the Flow Length tool in Spatial Analyst's Hydrology Toolbox to find the Longest Flow Path

ThomasDilts — Mon, 04 Feb 2019 17:12:12 GMT

Hi Luke. Thanks for pointing that out. I've updated the zip file so that it is no longer empty.