Erode Users Manual (a DOS-based IDRISI module)
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.
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
The program will then ask for the following input and output filenames.
- The name of the DEM file to be processed. This file must be in the subdirectory
specified for data in the idrisi.env file.
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.
- Slope filename - The name of the output file to contain the slope angle data.
- Slope Length filename - The name of the output file to contain the slope length data.
- Cumulative slope length filename - The name of the output file to contain the cumulative
slope length data.
- Outflow filename. This represents the flowdirection (or aspect) of each cell.
- 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
- 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
- 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.
- 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.
- 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
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