[GRASS-dev] [GRASS GIS] #2347: r.ros uses arbitrary direction convention

[GRASS GIS]

#2347: r.ros uses arbitrary direction convention
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 Reporter:  madi                     |       Owner:  grass-dev@…              
     Type:  defect                   |      Status:  new                      
 Priority:  normal                   |   Milestone:  6.4.4                    
Component:  Default                  |     Version:  unspecified              
 Keywords:  r.ros, r.spread, angles  |    Platform:  Unspecified              
      Cpu:  Unspecified              |  
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Comment(by hamish):

 fwiw flow direction from/to is often called meteorological versus
 oceanographic convention, different ones are used in different fields of
 study, which is fun when dealing with coupled wind-ocean models. (but not
 as frustrating as GRIB wind data not noting if its u,v wind velo
 components are relative to grid north or geographic north! remember to
 make that correction too: `g.region -n` and the r.convergence_angle addon)

 also I think it is useful to call the "GRASS" convention 'theta CCW from
 the +x axis' (i.e. mathematical|Cartesian convention), instead of 'CCW
 from east'. Most people will not know what the "GRASS way" means, but they
 will know what Cartesian theta will be like.
 (all this versus the navigational convention of degrees CW from north)


 In the d.barb module (G6 addons) I try to cover all ways, polar and
 component too:
 {{{
 Flags:
   -r   Rotate direction 180 degrees
         Useful for switching between atmospheric and oceanographic
 conventions
 ...

 Parameters:
         direction   Raster map (or attribute column) containing velocity
 direction
         magnitude   Raster map (or attribute column) containing velocity
 magnitude
                 u   Raster map (or attribute column) containing
 u-component of velocity
                 v   Raster map (or attribute column) containing
 v-component of velocity
 ...
       aspect_type   Direction map aspect type
                     options: cartesian,compass
                     default: cartesian
 }}}


 best to try some obvious cases with strong wind from one direction to
 double check assumptions, and always note in the option descriptions which
 one is used. And hopefully there is a prevailing wind direction where you
 work to double check against.


 For r.ros I can understand some logic in wind direction using the
 meteorological convention ("from", a northerly wind blows from the north),
 while maximum spread output being "to" as in that's the direction the
 front is traveling.

 Anyway, for backwards compatibility it should only be well documented in
 G6, not changed.


 Hamish


 ps, to convert: `r.mapcalc "cartesian = 90 - compass"`, or vice versa;
 from/to is just +180.

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Ticket URL: <https://trac.osgeo.org/grass/ticket/2347#comment:2>
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