Understanding percentage drop-raster (flow direction-tool)

I'm creating a Topographic Wetness Index using a python script (http://arcscripts.esri.com/details.asp?dbid=16750)

As I don't want this just to be a blackbox, I'm trying to understand the different rasters this script produces. One of them is outdrop-raster, which is a by-product of flow direction raster. According to ESRI:

"The output drop raster is calculated as the difference in z-value divided by the path length between the cell centers, expressed in percentages. 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. The result is a map of percent rise in the path of steepest descent from each cell."

..which seems reasonable. The problem is, that there are several numbers which pop out. For example 16,666.. % and 25 % are by far the most common numbers, why? I'm sure there is an explanation for this, but I just can't figure it out. It doesn't seem logical to have continuous elevation (thus slope) data and results that are not (that) continuous. Original data is low-density LiDAR DEM (2 m pix), values are 32 bit float (197.132, 198.013 m etc.). 5 by 5 example, darker gray equals flow direcetion to lower left and percentage 16,666 % drop while lighter gray here has 25 % drop and flow direction to the left

• Is your projection in the same units as your Z values? – Scro Oct 25 '13 at 8:09
• If the numbers calculated by the software algorithm do not convince you, why not manually calculating a few values between cells and then comparing them with those given by the algorithm? – multigoodverse Oct 25 '13 at 9:32
• I Already calculated some flow direction values, and they seem fine. It's more difficult (or time consuming) to calculate paths and path lenghts. I was just hoping if somebody had an idea why these values tend to stack up. – reima Oct 25 '13 at 9:53
• All units are in metres. – reima Oct 25 '13 at 9:53
• @reima I don't think Ardit Sulce was suggesting that you calculate path lengths, rather to verify slope between cells. Subtract the Z value from any cell from the Z value its neighbor with the greatest elevation difference. Then divide that by 2 if the cells are adjacent, or by 2.828 if they are diagonal. You won't get the exact value of the drop raster, but most problems with units will result in errors that differ in order of magnitude. Let us know what you find. – Scro Oct 25 '13 at 13:10