# Using the Raster calculator to determine slope length

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04-11-2016 05:00 PM
New Contributor III

Hallo All, I am trying to use the raster calculator to calculate Slope length SL for my RUSLE model. All the necessary layers as inputs have been prepare including flow accumulation, slope in percentage. But when iinput the formula (see attached) and run the map algebra tool i receive an error message. Any ideas where i may be missing it?

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11 Replies
MVP Esteemed Contributor

drop the LS ==  that is checking for equality, your output will be named as given in the output line

use sqrt ... not sure if ^0.5 works in all places... but be safe.

why don't you split stuff and do the calculations in baby steps to ensure that each component works?

you can then delete the intermediate steps or combine them into one smokin' one-liner if the pieces work.

PS move this to Spatial Analyst​ it has nothing to do with coordinate reference systems

MVP Honored Contributor

What is LS == supposed to mean?

Also, I suspect you're going to want to change ^ to ** (Map Algebra operators).

New Contributor III

@Darren LS represent Slope length and is one important input in the RUSLE model

MVP Esteemed Contributor

There many links on this site, albeit, 3-5 years old on LS ... search using ... usle ... as a search key, for more background and links

New Contributor III

checked but found only one, and that was on model building, but thanks. Will follow your earlier suggestion to do it peace meal and see what happens.

MVP Esteemed Contributor

Considering the output of the expression in Raster Calculator is Slope Length (LS), you need to do following two steps

1) Delete LS== from the expression. (You do not mention the LHS in the expression here in Raster Calculator)

2) Replace ^ with ** for square operation.

MVP Esteemed Contributor

covered way above... and sqrt would be better

New Contributor III

@Dan, i have done it in bits as you suggested and it worked, but i am confused with "sqrt" suggested. below is the equation i am using and m is 0.5, so in fact i used the boolean sign, hope i was right

New Contributor III

This was how the whole thing was run; the final ourput SL was the product of rastercalc1 and rastercalc 2