[GRASS-user] A question before I embark on a programming exercise

[Moritz Lennert]

On 12/11/14 05:10, Thomas Adams wrote:
> I need to generate an ascii text file from a flow direction grid that
> consists of (among a couple other things that don't really matter at
> this point) for each pixel:

Rainer has already given you a whole program, but here's a decomposition 
in simple (untested) r.mapcalc steps:

>
> (1) a unique integer identifier (1 -- N) for the pixel

r.mapcalc "id = row()*N + col()" # where N = nb cols * 10

(you might have to watch out for integer overflow, though)

> (2) the integer identifier of the downstream pixel (assuming there is
> ONLY one)

Calculate flow direction in SFD mode and use something like this:

r.mapcalc down_id = if(dir=1, id[1,1], if(dir=2, id[0,1], if(dir=3, 
id[-1,1] .... etc


> (3) the x,y location of the pixel (presumably, the lower left corner of
> the pixel)

center of pixel:
r.mapcalc x = x()
r.mapcalc y = y()

if you want the lower left corner:

r.mapcalc x = x() - ewres()/2
r.mapcalc y = y() - nsres()/2


Then export everything with

r.stats -1 in=id,x,y,down_id

Moritz