I wanted to calculate the LS Factor of the USLE (Universal Soil Loss Equation) with the tool Raster Calculator in ArcGis 10.

First, I filled my DEM with the tool Fill. (DEM was calculated in UTM)

Second, I calculated the Flow Direction with the Tool Flow Direction.

Third, I calculated the Flow Accumulation with my Flow Direction as Inputfile. [ATTACH=CONFIG]16862[/ATTACH]

Fourth, I calculated the Slope in Degrees by the Spatial Analyst tool Surface and Slope. [ATTACH=CONFIG]16863[/ATTACH]

Fifthly, I used the Equation by Moore and Burch 1986: LS = (Flow Accumulation * Cell Size/22.13)^0.4 * (sin (slope/0.0896))^1.3

Sixthly: I typed in the raster calculator the equation:

(Power((("FlowAcc" * 92.46745509) / 22.13),0.4)) * (Power((Sin("Slope_Degree" / 0.0896)),1.3))

But my result look very very strange! There a lot of white areas! The Raster is not like cover areas.... It looks like that every calculated value is seperated... It looks like a dotted map....What did I wrong?? Did I forget something?? I can`t believe that this is right??!! [ATTACH=CONFIG]16864[/ATTACH]

First, I filled my DEM with the tool Fill. (DEM was calculated in UTM)

Second, I calculated the Flow Direction with the Tool Flow Direction.

Third, I calculated the Flow Accumulation with my Flow Direction as Inputfile. [ATTACH=CONFIG]16862[/ATTACH]

Fourth, I calculated the Slope in Degrees by the Spatial Analyst tool Surface and Slope. [ATTACH=CONFIG]16863[/ATTACH]

Fifthly, I used the Equation by Moore and Burch 1986: LS = (Flow Accumulation * Cell Size/22.13)^0.4 * (sin (slope/0.0896))^1.3

Sixthly: I typed in the raster calculator the equation:

(Power((("FlowAcc" * 92.46745509) / 22.13),0.4)) * (Power((Sin("Slope_Degree" / 0.0896)),1.3))

But my result look very very strange! There a lot of white areas! The Raster is not like cover areas.... It looks like that every calculated value is seperated... It looks like a dotted map....What did I wrong?? Did I forget something?? I can`t believe that this is right??!! [ATTACH=CONFIG]16864[/ATTACH]

At first you have to investigate your map. Are these white areas NoData? Which values are in other layers where NoData values appear in LS layer. It seems that there is a problem with FlowAccumulation layer - there also seem to have NoData values.

I have used Wischmeier and Smith's LS factor formula and it works:

Pow(([FlowLength]/22.13), [m_factor] / 10)*(((Sin([Slope_degrees] / deg)*Sin([Slope_degrees] / deg))*65.41)+(Sin([Slope_degrees] / deg)*4.56)+0.065)

FlowLength substitues slope length, like FlowAccumulation

m factor is 0,2 for slopes less than 0.01% (slope_percent), 0,3 for slopes 0,01-0,035, 0,4 for slopes 0,035-0,05 and 0,5 for slopes over 0,05%. As the reclassify tool does not enable to give new values like 0,2 then just integer values (like 2) have to assigned and these vales are divided by 10 in formula (m_factor / 10)

deg - constant, 57,296

I hope this helped,

Anne