I have derived Topographic Wetness Index (TWI)/CTI with ArcGIS 10.3 using 30m DEM resolution. Earlier, it was propagating negative values which were mainly confined to ridges. I have restructured the 'Raster Calculator' expression as below and got rid of negative values.

Ln(("Flow_acc"+ 1 ) * 900/"Tan_SlopeRad")

The below process diagram would acquaint you about steps I have taken to arrive at the positive integer values. Process Flow Diagram

and the resultant image clearly indicates values having 'No Data'. These values are mainly confined to Zero Degree Slope within River. Resultant TWI Image

Now, How would I deal with values having ''No Data'???


2 Answers 2


Zero slope will lead to an infinitely high TWI value, as described in the answer to a similar question (Getting Negative Topographic Wetness Index (TWI) values in SAGA GIS?). This is what leads to the NoData values you see. As far as I know, TWI is used as an indicator for wetness conditions in soils, so probably looking at locations right within a river bed won't make sense anyway.


Add a small value such as 0.0001 to the "Tan_SlopeRad" raster to eliminate any undefined values produced by ln(0). This is the same reason you add one to the flow accumulation.

Ln(("Flow_acc"+ 1 ) * 900/("Tan_SlopeRad")+ 0.0001)

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