[GRASS-dev] Re: r.walk and r.drain improvements

[Dylan Beaudette]

On Tue, Jun 24, 2008 at 6:06 AM, Hamish <hamish_b at yahoo.com> wrote:
> Dylan:
>> > Interesting post Colin. Can you comment on the differences between
>> > r.drain and r.walk in this example [1], in light of your findings?
>> > 1. http://casoilresource.lawr.ucdavis.edu/drupal/node/544
>
> teaching r.walk to follow ridgelines when possible would be cool for back country stuff. Perhaps r.mapcalc multiply the slope-cost input map with a r.param.scale feature map that likes ridges and saddles but doesn't like gullies and pits? treelines too.

That is a good idea. I have done something similar in the past [1] ,
with vegetated areas / lakes, to 'force' the cost surface in ways
beneficial to hiking. Vegetated areas were made easier to traverse
(closed canopy pine forests) and lakes were made impossible to
traverse. However, adding more of this kind of intuition via landform
element would be a great feature.

1. http://casoilresource.lawr.ucdavis.edu/drupal/node/244


>
> Also set cost map to NULL if slope > x so it doesn't have you crossing cumulatively short but physically challenging 20m cliffs.
>

Ah... that is what my example above is missing. I didn't know that
r.drain would go around NULL cells!

Great tips.

Dylan


> Colin:
>> Excellently documented example by the way.
>>
>> The path is probably quite similar but the point is that there is
>> currently no way to ensure that the r.drain path conforms to the
>> same path as the optimal path of cost accumulation (calculated
>> by r.walk or r.cost).
>
> AFAIR r.drain just blindly climbs to the next up/downhill D8 cell, in a loop, until it can climb/drop no more. thus it is not "least" cost at all, just one valid solution?  (??)
>
>
> Hamish
>
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>
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