The Flow Direction tool creates a raster of flow direction from each cell to its downslope neighbor, or neighbors, using the D8, Multiple Flow Direction (MFD), or D-Infinity (DINF) method.
The output is a hosted imagery layer.
Learn more about how Flow Direction works
Usage notes
Flow Direction includes configurations for input layer, flow direction settings, and result layer.
Input layer
The Input layer group includes the following parameter:
The Input surface raster is the elevation raster that will be used for calculation.
Flow direction settings
The Flow direction settings group includes the following parameters:
Flow direction type specifies the type of flow method that will be used when computing flow direction.
D8—The D8 method assigns flow direction to the steepest downslope neighbor calculated by taking the difference in z-value divided by the path length between cell centers (1 for cardinal cells and the square root of 2 for diagonal cells). The values in the Output flow direction raster name layer will be integer values in the range of 1 to 255. The values from the center of each direction are specified in the following diagram:
If a cell has the same change in z-value in multiple directions, the D8 flow direction will be undefined. In this case, the value for such cell will be the sum of the possible directions.
This is the default.
MFD—The MFD flow method partitions the flow direction across downslope neighbors according to an adaptive partition exponent. The adaptive component is estimated as a function of maximum slope gradient, which considers local terrain conditions (Qin et al., 2007). The values in the Output flow direction raster name layer will be integer values in the range of 1 to 255, showing the predominant flow direction (toward the cell that receives the largest fraction of flow according to the partition scheme) for ease of interpretation.
However, an MFD flow direction output raster is an input recognized by the Flow Accumulation tool that will use the MFD flow directions to proportion and accumulate flow from each cell to all downslope neighbors.
DINF—The DINF method determines flow direction as the steepest downward slope on eight triangular facets formed in a 3 by 3 cell window centered on the cell of interest. Flow direction output is a floating-point raster represented as a single angle in degrees counterclockwise from 0 (due east) to 360 (again due east).
The Force edge cells to flow outwards parameter specifies whether edge cells will always flow outward or follow normal flow rules.
- Unchecked—The flow direction will be toward the inner cell with the steepest drop in z-value. However, if the drop is less than or equal to zero, the cell will flow outward from the surface raster. This will be the same for all cells across the raster. This is the default.
- Checked—The flow direction at edge cells will always flow outward from the surface raster.
Result layers
The Result layers group includes the following parameters:
Output flow direction raster name is the name of the output raster that will contain the flow direction result.
The name must be unique. If a layer with the same name already exists in your organization, the tool will fail and you will be prompted to use a different name.
The Optional layers group includes the following parameter:
Output drop raster name is the name of the output raster that will contain the drop raster result.
The name must be unique. If a layer with the same name already exists in your organization, the tool will fail and you will be prompted to use a different name.
- Save in folder specifies the name of a folder in My content where the result will be saved.
Environments
Analysis environment settings are additional parameters that affect a tool's results. You can access the tool's analysis environment settings from the Environment settings parameter group.
This tool honors the following analysis environments:
- Output coordinate system
- Processing extent
Note:
The default processing extent in Map Viewer is Full extent. This default is different from Map Viewer Classic in which Use current map extent is enabled by default.
- Snap raster
- Cell size
- Mask
- Parallel processing factor
Outputs
This tool includes the following outputs:
The Output flow direction raster name layer stores the value of flow direction at each cell.
The output flow direction raster is of integer type. If MFD was specified for the Flow direction type parameter, the value of each cell shows the predominant flow direction, which is toward the cell that receives the largest fraction of flow according to the partition scheme.
The Output drop raster name layer stores the value of the percentage of the ratio between the maximum elevation change and the path length (between cell centers) along the direction of flow. The output drop raster is of floating-point type. For adjacent cells, this is analogous to the percent slope between cells. Across a flat area, the distance becomes the distance to the nearest cell of lower elevation.
Usage requirements
This tool requires the following licensing and configurations:
- Creator or GIS Professional user type
- Publisher or Administrator role, or an equivalent custom role
- Available when the organization is configured for raster analytics with a premium capability license
References
Jenson, S. K., and Domingue, J. O. 1988. "Extracting Topographic Structure from Digital Elevation Data for Geographic Information System Analysis." Photogrammetric Engineering and Remote Sensing 54 (11): 1593–1600.
Qin, C., Zhu, A. X., Pei, T., Li, B., Zhou, C., & Yang, L. 2007. "An adaptive approach to selecting a flow partition exponent for a multiple flow direction algorithm." International Journal of Geographical Information Science 21(4): 443-458.
Tarboton, D. G. 1997. "A new method for the determination of flow directions and upslope areas in grid digital elevation models." Water Resources Research 33(2): 309-319.
Resources
Use the following resources to learn more:
- How Flow Direction works
- Flow Direction in ArcGIS REST API
- flow_direction in ArcGIS API for Python
- Flow Direction in ArcGIS Pro with the Raster Analysis extension
- Flow Direction in ArcGIS Pro with the Spatial Analyst extension