The Input surface raster is the elevation raster that will be used for calculation.

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.

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.