[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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Changes (by wenzeslaus):

  * keywords:  r.ros => r.ros, r.spread, angles


Comment:

 There is not only wind direction but also direction of maximal rate of
 spread and aspect.

 G7:r.ros:

  * ''direction'': wind directions (degree), CW from N
  * ''aspect'': aspect (degree, CCW from E) - GRASS convention, not tested
  * ''output'': `.maxdir`: directions of maximal ROS, CW from N
  * ''velocity'': wind velocities at half of the average flame height
 (feet/minute)

 G7:r.spread:

  * ''dir'': directions of maximal ROS (degree), CW from N
  * ''w_speed'': midflame wind speed (ft/min)

 [http://en.wikipedia.org/wiki/Wind_direction Wikipedia says]:

 > Wind direction is reported by the direction from which it originates.
 For example, a northerly wind blows from the north to the south. Wind
 direction is usually reported in cardinal directions or in azimuth
 degrees. For example, a wind coming from the south is given as 180
 degrees; one from the east is 90 degrees.

 Which is CW from N but defining the direction ''from'' wind blows, not the
 direction of ''vector field'' defined by wind directions and wind speeds.

 This is also what [http://grasswiki.osgeo.org/wiki/Meteorology#Conventions
 GRASS Wiki says] about "main wind data providers". (If you put together
 the images and the text you get that:) GRASS convention is CCW from E and
 pointing ''to'' the direction (aspect, flow) while wind is usually CW from
 N and pointing ''from'' where wind blows.

 What is unclear to me why the GRASS Wiki says that GRASS convention is 0
 for vector of zero magnitude (e.g. no wind) and 360 for the reference axis
 direction. I don't remember this behavior from G7:r.slope.aspect,
 G7:d.rast.arrow, or G7:r.flow. But this is not particularly important now.

 Again, `r.ros` is using wind direction clockwise from north. And it is
 right in its own sense since direction is pointing ''to'' but as Madi
 says, it is not a GRASS nor meteorological convention. However, it is not
 uncommon convention, it is an azimuth of wind (''to'') direction (e.g.
 ArcGIS defines aspect in the same way).

 I seems that there is several applicable conventions for wind. Perhaps, we
 should support more than one. Supporting the GRASS one is good for
 consistency (and other usages of `r.ros` and `r.spread` than fire), the
 meteorologic one can be practical and the current is useful default for
 backward compatibility.

 Also, the maximum ROS direction map passed from `r.ros` to `r.spread`
 follows `r.ros`'s wind direction convention, so it is wrong from GRASS
 point of view. We should probably change this too, although it is
 basically not part of the interface, so it is not critical.

 I was not looking to the source code, so I don't know how difficult it
 would be to change this (e.g., are angles out of ``[0, 360]`` interval
 allowed?).

-- 
Ticket URL: <https://trac.osgeo.org/grass/ticket/2347#comment:1>
GRASS GIS <http://grass.osgeo.org>