[GRASS-user] r.terraflow

Tue Jul 29 07:35:32 EDT 2008

Ivan et al,

Below are some snippets from an email thread in 2005:

Has anyone had any luck using the results from r.terraflow in 
r.water.outlet? More specifically I want to use the output direction 
grid from r.terraflow for the drainage direction map input of 
r.water.outlet. The r.water.outlet documentation only refers to using a 
drainage direction map generated in r.watershed.

I really don't know about r.water.outlet but while r.whatershed generate
flow direction with a D4 or D8 algorithm while r.terraflow use a Dinf
algorithm (6 month ago the -s flag was not working, maybe now is fixed,
maybe not.... try to test....).

r.terraflow uses by default a MFD algorithm (multiple flow path). In short, 
the outflow of one cell is distributed among ALL its downslope neighbors, 
according to the relative differences in elevation. The nb stored in the flow 
direction output of terraflow corresponds to the neighbors id, stored as an 
octet (so, between 0 and 255), clockwise from East (1 for East, 2 for SE, 4 
for S, 8 for SW...) : for example, if water flows to the North and the south 
simultaneously, the flow direction value is 68 (4 for S + 64 for N). NNE ? 
64+128, and so on...

r.watershed uses a SFD (single flow direction) algorithm, where water is 
routed to ONE downslope neighbor. The flow directions are coded as an integer 
between 1 and 8.

There should be a trick to get main flow directions (pseudoSF) from terraflow 
MFD: check the sources, direction.cc: there's a series of test to get the 
main direction from the 0-255 value (function findDominant). A simple script 
should do the work. I tried to write such a script smoe time ago, but don't 
ask me where it is, my box is a mess these days...

OK, that makes sense. If I use the -s flag to force r.terraflow to use 
SFD will my flow directions be 1-8 rather than 0-255?

Nope, the values will still be between 0 and 128, but as 1,2,4,8,16,32,64,128 
only: you'll still have to postprocess your data to get it to a number 
between 1 and 8. And make sure that the way the directions are reported in 
r.watershed are consistent (viz, 1 is E and you go clockwise)

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. 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). 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.

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)

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

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.

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

I hope this helps…

Regards,
Tom

ivan marchesini wrote:
> Il giorno mar, 29/07/2008 alle 02.22 +0200, G. Allegri ha scritto:
>   
>> Thanks Luca.
>> The coding mapping seems correct. So, zeroes are for unsolved pixels...
>> I'll do some tests to compare the results with r.watershed. At now
>> they seem quite different. I'm not sure if I should trust onflow
>> directions from r.terraflow to use them in r.water.outlet...
>>
>>     
>
> hmmmm.... in my opinion the r.terraflow output is not usefull for
> r.water.outlet but if you find a solution please let we know   ;-)
>
>  bye
>
> Ivan
>
>
>
>
>   
>> http://lists.osgeo.org/mailman/listinfo/grass-user
>>
>>     

-- 
Thomas E Adams
National Weather Service
Ohio River Forecast Center
1901 South State Route 134
Wilmington, OH 45177

EMAIL:	thomas.adams at noaa.gov

VOICE:	937-383-0528
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