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When you perform a flow accumulation on a DEM, the upslope area is defined as all of the inflowing grid cells AND the cell itself. Thus, a grid cell is actually part of its own contributing area. The minimum upslope area (not specific contributing area) is equal to the area of one grid cell, i.e. a grid cell with no inflow cells. I think this is the reason ...


This is a good question, and one that I tend to get asked from time to time. First, as you've pointed out, the equation for TWI = ln(a / tan(B)), where a is the 'specific' catchment area (i.e. the upslope inflowing area normalized for a measure of contour length) and B is the slope gradient, in radians, at the grid cell. As you correctly pointed out TWI will ...


A good way to do this analysis would be create a TIN from your points then calculate the contours. You can do this in GIS software or even by hand.

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