Calculate Flow Direction

Flow direction, that is, the flow direction of water on the hydrological surface. Calculate Flow Direction is one of the Crucial Steps of Hydrological Analysis. Many functions of Hydrological Analysis need to be based on Direction Raster, such as calculating Cumulative Flow Accumulation, Calculate Flow Length and watershed.

In SuperMap, the 8 neighborhood grids of the center grid are encoded. The encoding is actually to take the power value of 2, and the encoding values are respectively the power values of 0, 1, 2, 3, 4, 5, 6, and 7 of 2, that is, 1, 2, 4, 8, 16, 32, 64, and 128, clockwise from the right grid of the central grid. The water flow directions of the central grid unit are east, southeast, south, southwest, west, northwest, north and northeast. Refer to the figure below. The flow direction for each central grid is determined by one of these eight values. For example, if the flow direction of the center grid is west, the flow direction is assigned a value of 16; if it is east, the flow direction is assigned a value of 1.

Figure: Flow direction diagram

Instructions for use

Boundary Flow Treatment

Cells on the boundary of the grid are special (they are on the boundary and there are less than eight possible flow directions). You can specify that the flow direction is outward. In this case, the flow direction value of the boundary grid is shown in the left figure below. Otherwise, the cells on the boundary will be assigned to no value (as shown in the right figure).

Figure: Schematic diagram of boundary grid flow direction

Gradient Raster

In Calculate Flow Direction, Application uses the maximum gradient method. This method calculates the steepest descent direction of the cell as the direction of the flow. The ratio of the elevation difference to the distance between the center cell and the adjacent cell is called the elevation gradient.

Flat Area Flow Processing

When calculating the flow direction of the flat area, that is, when the elevation change gradient is 0, the Application uses the fill height operation to allow the points without flow direction in the flat area to flow out through the fill height. The height filling operation is based on the elevation value of the surrounding area. The steepest descending slope is calculated from the data after height filling, and the direction with the steepest slope is taken as the flow direction.

Function entrance

  • Spatial Analysis tab- Raster Analysis group-> Hydrological Analysis-> Calculate Flow Direction.
  • Toolbox-> Raster Analysis-> Hydrological Analysis-> Calculate Flow Direction.

Parameter Description

  • Source Data: Set the Datasource and Dataset where the DGM data to be Calculate Flow Direction is located.
  • Force the flow direction outwards if it is at edge: The flow of water in all grids on the grid surface is outward.
  • Create the changing elevation gradient: Select this item. When Calculate Flow Direction, the gradient change of each grid elevation will be calculated at the same time, and a Gradient Raster will be output. It is not selected by default, that is, the elevation Gradient Raster is not created.
  • Result Data: Set the name of Datasource, Direction Raster and Gradient Raster to save the result. Note: Gradient Raster names can only be entered when "Create the changing elevation gradient" is selected.
  • Click the Execute button to execute the prepared analysis function. After the execution completed, the Output Window will prompt whether the Result succeeds or fails.