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

[Margherita Di Leo]

Rich,

On Fri, Jun 22, 2012 at 6:37 PM, Rich Shepard <rshepard at appl-ecosys.com>wrote:

>  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.
>

The prevalent orientation is defined as the orientation of the directing
vector. The directing vector is a segment which joins the basin's
barycenter and the outlet.
The orientation is given by the formula:
prevalent_orientation = math.atan(delta_y/delta_x)

where

delta_x = abs(float(basin_east) - east)
delta_y = abs(float(basin_north) - north)

being basin_east and basin_north the coordinates of barycenter and east and
north those of the outlet.

HTH,
madi


-- 
Dr. Margherita Di Leo
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