[GRASS-dev] [GRASS GIS] #2637: Get direction raster in clockwise degrees starting from the North

GRASS GIS trac at osgeo.org
Sun Jan 28 22:27:45 PST 2018

#2637: Get direction raster in clockwise degrees starting from the North
  Reporter:  cgravelm     |      Owner:  grass-dev@…
      Type:  enhancement  |     Status:  new
  Priority:  normal       |  Milestone:  7.4.1
 Component:  Raster       |    Version:  7.0.0
Resolution:               |   Keywords:
       CPU:  Unspecified  |   Platform:  MacOSX

Comment (by cmbarton):

 Such a flag would still be useful for a variety of applications and should
 not interfere with other modules if the default behavior remains the same.
 Note that in r.slope.aspect there is the choice of outputting slope as
 degrees or percent, with degrees being the default.

 Just because outputting direction as in a compass bearing is not useful
 for some people does not mean that it would not be useful for other. For
 anyone interested in knowing what compass direction a location is facing
 (and that is a question that people have), the current output is
 completely misleading.

 The manual states "The aspect output raster map indicates the direction
 that slopes are facing". To most people, the word "direction" refers to a
 compass direction = azimuth. If they keep reading, it gets increasingly
 baffling from this point of view. "The aspect categories represent the
 number degrees of east. Category and color table files are also generated
 for the aspect raster map. The aspect categories represent the number
 degrees of east and they increase counterclockwise: 90 degrees is North,
 180 is West, 270 is South 360 is East."

 I understand the history of this (from before GRASS rasters were
 georegistered) and realize that because of this long legacy effect, other
 raster modules have been written to expect this kind of non-cartographic
 direction metric. But a simple flag to let users change this to represent
 what a map direction is commonly expected to mean seems like a useful

Ticket URL: <https://trac.osgeo.org/grass/ticket/2637#comment:21>
GRASS GIS <https://grass.osgeo.org>

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