# How do I write my equation with conditions into the QGIS raster calculator?

I have two raster layers - one with slope(degrees), called theta, and one with slope(%). I need to calculate the S-factor by using attatched formula. How do I write this in the raster calculator? Is it possible? ## 1 Answer

A "True" condition evaluates numerically to 1, and a "False" condition to zero, so if you add up the result of multiplying the test conditions by the value expressions for those tests, you get the result you are after. So:

``````("slope@1" < 9 ) * (10.8 * sin("theta@1")+0.03) +
("slope@1" >=9 ) * (16.8 * sin("theta@1")-0.05)
``````

should do it. Make sure your theta is in radians, not degrees. To convert, multiply by pi/180, except there's no "pi" in the calculator, but `acos(0)` is pi/2, so multiply by `acos(0)/90` is equivalent.

[Hint: save your calculator expressions in a text file and paste them into/out of the calculator because it doesn't save them and I'm not typing that all in again!!]

• Thank you so much! Sorry, i'll save my calculator expressions from now on :-) – anneke May 6 at 16:01