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

[Colin Nielsen]

I have been working on the grass-6.3.0 release version which I
compiled from source myself.

Several problems:
1) I tried to run the unaltered r.drain on a cost surface made with an
unaltered r.walk and there were pits in it causing the r.drain to fail
to find the start point. Should that even be possible? How could there
be pits in a cost surface?

2) I just tried to do a simple comparison with r.cost and found that
it isn't functioning properly at all. It gets to "Finding cost path"
and starts counting percents. It works fine until 99% and then it
lists 100% many times, 101% many times and so on. It was around 250%
when I stopped it. Perhaps this is something in my current
compile....? I have no idea.

On Tue, Jun 24, 2008 at 5:04 PM, Michael Barton <michael.barton at asu.edu> wrote:
> I'm happy to see these improvements to r.drain. In this context I want to
> mention that r.drain doesn't seem to work correctly in latlon locations.
>
> Michael

I'll try to look into latlong but that may be beyond my current ability.

-Colin

>
>
> On 6/24/08 9:00 AM, "grass-dev-request at lists.osgeo.org"
> <grass-dev-request at lists.osgeo.org> wrote:
>
>> Message: 4
>> Date: Tue, 24 Jun 2008 07:27:37 -0700
>> From: "Dylan Beaudette" <dylan.beaudette at gmail.com>
>> Subject: Re: [GRASS-dev] Re: r.walk and r.drain improvements
>> To: hamish_b at yahoo.com
>> Cc: grass-dev at lists.osgeo.org
>> Message-ID:
>> <3c5546140806240727j168f291eh2ddde34c874e5890 at mail.gmail.com>
>> Content-Type: text/plain; charset=ISO-8859-1
>>
>> 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
>>>
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> grass-dev mailing list
>>> grass-dev at lists.osgeo.org
>>> http://lists.osgeo.org/mailman/listinfo/grass-dev
>>>
>>
>
> __________________________________________
> Michael Barton, Professor of Anthropology
> Director of Graduate Studies
> School of Human Evolution & Social Change
> Center for Social Dynamics & Complexity
> Arizona State University
>
> phone: 480-965-6213
> fax: 480-965-7671
> www: http://www.public.asu.edu/~cmbarton
>
>
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