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

[Johannes Radinger]

Hi Tom,

what comes into my mind is following approach (not tested):

1) calculate flow direction using r.watershed for each pixel of your map
2) transforming raster to vector points (r.to.vect), adding some columns
for your results (X, Y, ID). This step should provide you already with
unique IDs for your cells/points.
3) With v.to.db you can update the X  and Y coordinate of your cell center
and together with the raster resolution you can get the lower left corner
as desired.
3) By knowing the flow direction for each point you should be able to
calculate the X and Y coordinate of the downstream cell (adding or
substracting your cell dimensions from the cell center, correct for
diagonal flow). When you know X and Y of your downstream cell then you
should be able to also get the ID of the downstream cell (most probably a
SQL query I guess)

Maybe there are easier ways....

hope it works,
Johannes

On Wed, Nov 12, 2014 at 5:10 AM, Thomas Adams <tea3rd at gmail.com> wrote:

> All:
>
> 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:
>
> (1) a unique integer identifier (1 -- N) for the pixel
> (2) the integer identifier of the downstream pixel (assuming there is ONLY
> one)
> (3) the x,y location of the pixel (presumably, the lower left corner of
> the pixel)
>
> has anyone already done something like this?
>
> It's needed (along with some header information) as an input file for a
> gridded distributed hydrologic model, identifying the flow connectivity
> from pixel to pixel for streamflow routing purposes. I have already
> formulated conceptually how to do this, but if someone has already done
> such a thing, why reinvent the wheel??
>
> Any help is appreciated.
>
> Regards,
> Tom
>
>
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