todd93

Miscalculated topographic wetness index

Discussion created by todd93 on Oct 26, 2012
Latest reply on Oct 26, 2012 by jevans02
I am trying to use a flow accumulation (fac) raster and a slope (slope) raster to calculate the topographic wetness index (TWI) in raster calculator as:

Ln("fac"/Tan("slope"))

The resulting output values do not make sense relative to hand calculations. For example:

If...
   fac = 6 cells, slope = 26.379343 degrees

   TWI = 0.701 (ArcGIS raster calculator)
   TWI  = 2.493 (by hand)


Also, many of the output pixels are "no data".

Does anyone know what might be causing this discrepancy?

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