[GRASS-user] Interpreting legend for TCI map

Rich Shepard rshepard at appl-ecosys.com
Mon Oct 22 09:31:58 PDT 2018

Hi folks,

   The attached image is a TCI map output from r.watershed. The legend is in
units of meters. I'm having difficulty interpreting the distances indicated
by the legend per the description of the map in the r.watershed manual page:

     "Output tci raster map contains topographic index TCI, computed as ln(α
     / tan(β)) where α is the cumulative upslope area draining through a
     point per unit contour length and tan(β) is the local slope angle. The
     TCI reflects the tendency of water to accumulate at any point in the
     catchment and the tendency for gravitational forces to move that water
     downslope (Quinn et al. 1991). This value will be negative if α / tan(β)
     < 1."

   I don't understand the source of the tick numbers in the legend. Seems to
me that the map depicts relative amounts of water accumulation which vary
with precipitation amounts, contributing area, and slope. Would it be better
to leave off tick marks and distances on the legend since the range of
colors represents the range from low accumulation (yellow) to high
accumulation (dark blue)?


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