[GRASSLIST:6524] Re: r.water.outlet

[Giacomo Bertoldi]

Drainage direction needed by r.water.outlet:

N: this means that water is going to North ...

N: 2
N/E: 1
E: 8
S/E: 7
S: 6
S/W: 5
W: 4
N/W: 3


Giacomo

>>>> I'm having trouble coming up with the drainage direction input for
>>>> r.water.outlet. The documentation indicates that the input should
>>>> come from r.watershed but I'm working with large DEMs and
>>>> r.watershed is too  slow.

Try r.terraflow for large dataset.
>
> try on a small area and compare outputs.
>
>>>> It seems like I should be able to use flow direction from
>>>> r.fill.dir in AGNPS format but it doesn't work out. r.fill.dir
>>>> produces  flow direction as 1-8 with 1 being North. R.water.outlet
>>>> takes flow  direction input as 1-8 but I'm unsure of the if 1 is
>>>> North. I suspect 1  is East but I'd appreciate any ideas.
>
>
> for the record, here's some aspect map format information collected 
> from
> the help pages and source code:
>
> (use d.rast.arrow and d.rast.num to investigate AGNPS, ANSWERS, GRASS
> formats)
>
>
> AGNPS (r.fill.dir)
> category values from 1-8, with 1 facing north and increasing values in
> the clockwise direction.
>
>
> ANSWERS (r.fill.dir)
> category values from 0-360 degrees, with 0 (360) facing east and values
> increasing in the counter clockwise direction at 45 degree increments.
>
>
> GRASS (r.fill.dir)
> broken in 6.0.0. (was old GRASS 4? format) In the old days 
> (pre-floating
> point GRASS rasters) there was a 24-points based GRASS aspect map 
> format
> which still rears its ugly head sometimes:
> 24,1,2   east
> 3,4,5    north east
> 6,7,8    north
> 9,10,11  north west
> 12,13,14 west
> 15,16,17 south west
> 18,19,20 south
> 21,22,23 south east
>
>
> GRASS (r.slope.aspect, others)
> categories represent the number degrees of east and they increase
> counterclockwise: 90deg is North, 180 is West, 270 is South and 360 is
> East. Beware: 0 may be null! Similar to the ANSWERS format AFAICT.
>
>
> r.param.scale
> the direction of maximum gradient (considered downslope) is stored as
> (West is 0 degree, East is +/- 180 degree):
>  * 0..+180 degree from West to North to East
>  * 0..-180 degree from West to South to East
> [Anomaly: I vote for changing this to standard GRASS5 aspect format]
>
>
> r.watershed drainage map (what r.water.outlet wants)
> drainage direction. Provides the "aspect" for each cell. Multiplying
> positive values by 45 will give the direction in degrees that the
> surface runoff will travel from that cell. The value -1 indicates that
> the cell is a depression area (defined by the depression input map).
> Other negative values indicate that surface runoff is leaving the
> boundaries of the current geographic region. The absolute value of 
> these
> negative cells indicates the direction of flow.
> [should we change this to standard AGNPS(+neg flag) aspect format ??]
>
>
> It would be nice to get these down to AGNPS, ANSWERS, and GRASS format
> anyway.
>
>
>
> so AGNPS is not the same as r.watershed format, but 45deg off:
>
> dir AGNPS  r.watershed
> N    1      0 or 8 ?
> NE   2      1
> E    3      2
> SE   4      3
> S    5      4
> SW   6      5
> W    7      6
> NW   8      7
>
>
> fix:
>
> r.mapcalc 'aspect.watershed = if(aspect.AGNPS == 1, 8, \
>   if(aspect.AGNPS > 1 && aspect.AGNPS <= 8, aspect.AGNPS-1, null() ))'
>
>
> if that works we can add it to the help page. (looks ok to me)
>
> note the negative values from the watershed program will be lost, which
> might adversely affect the output of r.water.outlet! (no idea)
>
>
>> I did try r.terraflow initially but in this case r.fill.dir is much
>> faster because all I need is flow direction, not flow accumulation,
>> TCI  or any of the other things r.terraflow provides.
>
> ok
>
>> Also, I was never able  to figure out how to get the flow direction
>> r.water.outlet needs from  the r.terraflow flow direction (I was the
>> guy that started that thread  you mentioned).
>
> Oh sorry, only short term memory here.
>
> FYI, I've whipped up a Matlab script to extract the dominant flow
> direction from the r.terraflow MFD output map and reclass it into
> the GRASS aspect format for use with d.rast.arrow, etc. It should
> be easy enough to rewrite it in something more Free and convert the
> export into any of the above aspect formats by changing asp_mtx[].
> Usually you can just use r.slope.aspect directly as a proxy.
>
> find it here:
>  http://grass.gdf-hannover.de/twiki/bin/view/GRASS/GrassAddOns
>
>
>> Regardless of what I use to get flow direction it would  be helpful to
>> know what format r.water.outlet  expects, ie what  direction  is 1
>> North, East or something else.
>
> AFAICT: 1 is NE, moving around the compass clockwise until 8 is North,
> and mind the negative values and information lost by taking the 
> absolute
> value of flow direction.
>
>
>
> Hamish
>
>