topic Re: Using the Raster calculator to determine slope length in Coordinate Reference Systems Questions

Using the Raster calculator to determine slope length

Kofi_Lartey — Tue, 12 Apr 2016 00:00:07 GMT

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?

Re: Using the Raster calculator to determine slope length

DanPatterson_Retired — Tue, 12 Apr 2016 00:17:18 GMT

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

Re: Using the Raster calculator to determine slope length

DarrenWiens2 — Tue, 12 Apr 2016 00:17:55 GMT

What is LS == supposed to mean?

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

Re: Using the Raster calculator to determine slope length

Kofi_Lartey — Tue, 12 Apr 2016 07:52:40 GMT

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

Re: Using the Raster calculator to determine slope length

DanPatterson_Retired — Tue, 12 Apr 2016 09:27:20 GMT

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

Re: Using the Raster calculator to determine slope length

Kofi_Lartey — Tue, 12 Apr 2016 19:32:36 GMT

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.

Re: Using the Raster calculator to determine slope length

JayantaPoddar — Tue, 12 Apr 2016 20:31:25 GMT

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.

Re: Using the Raster calculator to determine slope length

DanPatterson_Retired — Tue, 12 Apr 2016 20:37:36 GMT

covered way above... and sqrt would be better

Re: Using the Raster calculator to determine slope length

Kofi_Lartey — Wed, 13 Apr 2016 01:42:50 GMT

@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

Re: Using the Raster calculator to determine slope length

Kofi_Lartey — Wed, 13 Apr 2016 01:56:29 GMT

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

Re: Using the Raster calculator to determine slope length

DanPatterson_Retired — Wed, 13 Apr 2016 01:57:45 GMT

4**0.5 ... or ... 4^ 0.5 ... or sqrt(4) ... = 2 math/linguistic stuff...

just consider them dialect differences of words that mean the same thing.

Each discipline has their preferences, and certain "words" or "pronunciations" are prefered in certain circumstances.

This is intended to provide clarity and continuity of terms .... hah !! BS!! but I digress... as in life, one needs to become versed in the 'languages' used by ones discipline

Re: Using the Raster calculator to determine slope length

DanPatterson_Retired — Wed, 13 Apr 2016 02:14:47 GMT

that is correct as suggested, see the sqrt explanation I posted