I am calculating Topographic Wetness Index for a study area using SRTM DEM (90 and 30m).
I've seen several versions of TWI (CTI). I have been using: TWI = ln(As +1 / tanB) where As = catchment area, B = slope (degrees/radians??)
When I run this function, the output contains negative values (using slope in degrees and radians). Are negative values acceptable? Or does this indicate an error in the analysis??
I understand the output should be scaled to 1-10, but negative values seems counter intuitive. Could someone explain what negative values could mean (due to flat areas, no data values?)
On a side not, the calculations are being run on on a very large area, which overaps several watersheds. Is it appropriate to run TWI over such a large extent?