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

[GRASS GIS]

#2637: Get direction raster in clockwise degrees starting from the North
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  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
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Comment (by mmetz):

 Replying to [comment:27 hellik]:
 > Replying to [comment:26 mmetz]:
 > > Replying to [comment:24 hellik]:
 > > > Replying to [comment:22 mmetz]:
 > > > >Therefore I suggest to produce zero aspect for zero slope or if
 slope is < min_slope. NULL is then reserved for those cells where slope
 and aspect can not be computed. This also means that when converting CCW
 from East to CW from North, North must be encoded as 360 degrees, not 0
 degrees.
 > > >
 > > > what about to assign -1 for cells with zero slope or slope is <
 min_slope?
 > >
 > > This would change the current default behaviour with min_slope = 0 and
 aspect = 0 if slope = 0. Direction = 0 has an equivalent meaning in
 r.cost, r.walk, r.drain, r.path, r.watershed, r.stream.extract: it is a
 valid cell, there is no way away from here, stop routing here.
 > >
 > > Also, -1 can be interpreted as 359, just like 0 can be interpreted as
 360, no improvement there. IMHO, the corresponding aspect value for zero
 slope must be clearly documented, then it can be easily changed if need
 be.
 >
 > a short note how other GIS calculate aspect:
 >
 > https://gis.stackexchange.com/questions/238999/understanding-aspect-
 units-in-qgis

 no mentioning of aspect for flat areas.

 gdaldem uses -9999 or 0 with -zero_for_flat

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Ticket URL: <https://trac.osgeo.org/grass/ticket/2637#comment:28>
GRASS GIS <https://grass.osgeo.org>