[GRASS-user] Meaning of 'Prevalent Orientation' in r.basin

[Rich Shepard]

   The r.basin wiki page tells me, "The prevalent orientation: in GRASS the
aspect categories represent the number degrees of east and they increase
counterclockwise: (90deg is North, 180 is West, 270 is South 360 is East).
The aspect value 0 is used to indicate undefined aspect in flat areas with
slope=0. We instead calculate the orientation as the number of degree from
north, increasing counterclockwise."

   I have 7 sub-basins for which r.basin presents prevalent orientation
ranging from 0.1 to 1.5 degrees west of North. That isn't correct if this
parameter is the overall orientation of the main channel in the basin. For
example, sub-basin #32 has the main channel headwaters in the northwest and
the outlet in the southeast with reported prevalent orientation of
0.81195844521. If a compass was set on the map and degrees increased west of
North, the main channel direction from headwaters to mouth is approximately
225 degrees.

   Ergo, I must be mis-interpreting prevalent orientation in the r.basin
output and would appreciate having someone explain it to me.

TIA,

Rich

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