I'm trying to calculate slope, aspect and curvature from a DEM with the "Slope, Aspect, Curvature" tool provided by SAGA. To accomplish this, the tool gives different methods to calculate the slope, but the outputs differs according to the methods.

As example, Maximum Slope method (Travis et al. 1975) gives six outputs, but Least Squares method (Horn, 1981) just gives 3 outputs. Is this normal? Don't all the methods should give the same number of outputs?

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Not all methods provide the same outputs. For example Tarboton's (1997) maximum triangle slope does not provide curvature derivatives, only slope and aspect. The Travis method provides 6 derivatives (parameters) where Zevenbergen & Thorne provide 9 derivatives. Once a method is selected, you should be able to see, and define, the outputs associated with a given method in the tool interface. The outputs column will indicate the available derivatives and if <create> is in the column then it is a default output where <not set> indicates an optional output.

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