Erode Users Manual (a DOS-based IDRISI module)

Version 2

Name: erode2.exe.

To be placed in the same subdirectory as the idrisi.env and idrisi.exe files. NOTE, THIS PROGRAM WON'T RUN IN IDRISI32 - BUT DOES FOR OLDER WINDOWS AND DOS VERSIONS. Sorry, no plans to re-do and recompile. If memory serves, the newer versions do include a LS factor calculation button.

Program Description:

This program (erode2.exe) is a DOS-based IDRISI module designed to calculate cumulative downhill slope length from a DEM. In the process, it calculates and saves three intermediate files -- maximum downhill slope angle, slope length, and outflow direction (flowdirection or aspect).

To Run: type "erode2" at the DOS prompt while in the appropriate subdirectory.

The program will then ask for the following input and output filenames.


    1. The name of the DEM file to be processed. This file must be in the subdirectory specified for data in the idrisi.env file. The DEM must be in REAL ASCII format. Erode2 will also query the resolution line of the DEM's filename.doc file, so be sure that this is correct. If the resolution is unknown, erode2 will crash.
    2. Slope filename - The name of the output file to contain the slope angle data.
    3. Slope Length filename - The name of the output file to contain the slope length data.
    4. Cumulative slope length filename - The name of the output file to contain the cumulative slope length data.
    5. Outflow filename. This represents the flowdirection (or aspect) of each cell.
    6. Cutoff Slope - A value that represents the change in slope required to reset the cumulative slope length value to zero. If the slope decreases by more than the specified amount then there will be deposition rather than erosion and the cumulative slope length will be reset to zero. The value given for the Cutoff Slope should be between 0 and 1. For example, if the cutoff slope is set to 0.5 (50%) and the slope angle changes along flowdirection from 10 degrees to 4 degrees, the change is greater than 50%, so the cumulative slope length is set to zero.  Again, this is done to simulate areas of deposition.  A cutoff slope of 0 will result in no resets of slope angle, providing output which includes maximum slope lengths.  See Hickey, Smith, and Jankowski (1994) for a longer discussion of the cutoff slope (full reference on web page).


The output files are placed in the same subdirectory as the DEM (your working environment).

    1. Slope file - The code will calculate the maximum downhill slope angle from the DEM. The slope will, therefore, correspond exactly to the outflow direction (aspect) of the cell in question
    2. Slope Length file - Contains the slope length for each cell. In other words, the distance from the center of one cell to the center of the next along flowdirection.   In Version 2, the slope length is NOT calculated using the slope angle.  So, the length is the distance in x,y space, not x,y,z.  This corresponds to the requirements for both the USLE and the RUSLE.
    3. Cumulative Slope Length file - Contains the maximum cumulative slope length for each cell. The sum of the slope lengths along flowdirection -- reset in cases of deposition (see above description of cutoff slope) and, where there are converging flowpaths, the longer cumulative slope length takes precedence.
    4. Outflow file - Contains the flowdirection (or aspect) for each cell. This output direction is the same as the direction of maximum downhill slope angle.

North = 1

NE = 2

E = 3

SE = 4

S = 5

SW = 6

W = 7

NW = 8

Note:  Version 1 of the software can still be downloaded from this page.  The only differences between the versions are:

1.    The slope length (V.1) is calculated in x,y,z space, while in V.2, only x,y space is used.  Version 2 produces the output required for the USLE and RUSLE.

2.    The files created by erode.exe (V.1) are placed in the IDRISI subdirectory (where the idrisi.env and idrisi.exe files are found), NOT wherever all other data files are stored (call it a free feature, not a bug…..)