Flow Accumulation

Usage tips

Command line and Scripting

• The result of Flow Accumulation is a raster of accumulated flow to each cell, as determined by accumulating the weight for all cells that flow into each downslope cell.

• Cells of undefined flow direction will only receive flow; they will not contribute to any downstream flow. A cell is considered to have an undefined flow direction if its value in the <in_flow_direction_raster> is anything other than 1, 2, 4, 8, 16, 32, 64, or 128.

• The accumulated flow is based on the number of cells flowing into each cell in the output raster. The current processing cell is not considered in this accumulation.

• Output cells with a high flow accumulation are areas of concentrated flow and can be used to identify stream channels.

• Output cells with a flow accumulation of zero are local topographic highs and can be used to identify ridges.

• If the <in_flow_direction_raster> is not created with the FlowDirection command, there is a chance that the defined flow could loop. If the flow direction does loop, Flow Accumulation will go into an infinite loop and never finish.

Map Algebra

• Cells of undefined flow direction will only receive flow; they will not contribute to any downstream flow. A cell is considered to have an undefined flow direction if its value in the flow direction raster is anything other than 1, 2, 4, 8, 16, 32, 64, or 128.

• The accumulated flow is based on the number of cells flowing into each cell in the output raster. The current processing cell is not considered in this accumulation.

• Output cells with a high flow accumulation are areas of concentrated flow and can be used to identify stream channels.

• Output cells with a flow accumulation of zero are local topographic highs and can be used to identify ridges.

• If the flow direction raster is not created with the FlowDirection command, there is a chance that the defined flow could loop. If the flow direction does loop, the FlowAccumulation command will go into an infinite loop and never finish.

• Learn more about how to specify the input raster dataset in the Map Algebra expression of Raster Calculator.

ArcObjects

• Cells of undefined flow direction will only receive flow; they will not contribute to any downstream flow. A cell is considered to have an undefined flow direction if its value in the flow direction raster is anything other than 1, 2, 4, 8, 16, 32, 64, or 128.

• The accumulated flow is based on the number of cells flowing into each cell in the output raster. The current processing cell is not considered in this accumulation.

• Output cells with a high flow accumulation are areas of concentrated flow and can be used to identify stream channels.

• Output cells with a flow accumulation of zero are local topographic highs and can be used to identify ridges.

• If the flow direction raster is not created with the FlowDirection method, there is a chance that the defined flow could loop. If the flow direction does loop, the FlowAccumulation method will go into an infinite loop and never finish.

• The output from the FlowAccumulation method is a floating-point raster object.

• The output from the FlowAccumulationInt method is an integer raster object.

Command line syntax
An overview of the Command Line window
FlowAccumulation_sa <in_flow_direction_raster> <out_accumulation_raster> {in_weight_raster} {FLOAT | INTEGER}

Command line example

FlowAccumulation_sa C:/data/flowdir C:/data/flowaccu

Scripting syntax
About getting started with writing geoprocessing scripts
FlowAccumulation_sa (in_flow_direction_raster, out_accumulation_raster, in_weight_raster, data_type)

Script example

# FlowAccumulation_sample.py
# Description: 
#   Creates a raster of accumulated flow to each cell.
# Requirements: None
# Author: ESRI
# Date: Sept 6, 2005
# Import system modules
import arcgisscripting

# Create the Geoprocessor object
gp = arcgisscripting.create()

try:
    # Set local variables
    InFlowDirectionRaster = "C:/data/flowdir"
    OutAccumulationRaster = "C:/data/flowaccu"

     # Check out Spatial Analyst extension license
    gp.CheckOutExtension("Spatial")

    # Process: FlowAccumulation
    gp.FlowAccumulation_sa(InFlowDirectionRaster, OutAccumulationRaster)

except:
    # If an error occurred while running a tool, then print the messages.
    print gp.GetMessages()

Map Algebra syntax
FlowAccumulation(<dir_grid>, {weight_grid})

Map Algebra example

flowaccumulation(flowdir)
flowaccumulation(flowdir, rainfall)
flowaccumulation(flowdirection(elev), rainfall)

ArcObjects syntax
IHydrologyOp::FlowAccumulation (directionRaster As IGeoDataset, [weightRaster As Variant]) As IGeoDataset

IHydrologyOp::FlowAccumulationInt (directionRaster As IGeoDataset, [weightRaster]) As IGeoDataset

Note: The output raster of FlowAccumulation is of Floating Point type. The output of FlowAccumulationInt is of Integer type. FlowAccumulationInt is a member of esriSpatialAnalyst.IHydrologyOp2.

ArcObjects example

' Create the RasterHydrologyOp object
Dim pHydrologyOp As IHydrologyOp
Set pHydrologyOp = New RasterHydrologyOp

' Declare the input direction raster object
Dim pDirectionDataset As IGeoDataset

' Calls function to open a raster dataset from disk
Set pDirectionDataset = OpenRasterDataset("D:\SpatialData", "directionraster")

' Declare the output raster object
Dim pOutputRaster As IGeoDataset

' Calls the method
Set pOutputRaster = pHydrologyOp.FlowAccumulation(pDirectionDataset)