[GRASS-dev] small changes to r.walk

Thu Nov 13 13:16:23 EST 2008

Several weeks back, Helena gave a very good explanation of how r.walk  
actually works. It makes very good sense (see below). However, from  
this explanation, it is clear that lambda and a friction map should be  
*optional* rather than *required* as they are now.

The main part of r.walk calculates the time (in seconds) needed to  
walk across a landscape.

For any cell,

total time = (walking time in seconds to traverse the cell given its  
slope) + (lambda * friction map)

Lambda is a weighting coefficient to convert the friction map to units  
that match the costs due to slope (i.e., units in seconds normally).

If you want to calculate walking time to traverse a landscape that is  
based solely on the topography (i.e., slope), then you need a friction  
map with a value of 0; lambda can be anything.

So this should be optional. Currently, you need to create a 0 friction  
map and try to figure out what lambda should be in order to run  
r.walk. This is sort of pointless and can cause considerable confusion.

So, can lambda and friction map be changed to optional arguments for  
our upcoming releases?

Thanks
Michael
____________

Begin forwarded message:

> From: Michael Barton <michael.barton at asu.edu>
> Date: October 10, 2008 9:27:46 PM GMT-07:00
> To: Helena Mitasova <hmitaso at unity.ncsu.edu>
> Cc: grass developers <grass-dev at lists.osgeo.org>, Ullah Isaac <Isaac.Ullah at asu.edu 
> >, David.Quixal at uv.es, Sean Bergin <Sean.Bergin at asu.edu>, Moreno  
> Martín Andrea <andrea.moreno at uv.es>
> Subject: Re: [GRASS-dev] default for r.walk
>
> Thanks very much for this thorough explanation Helena. It is quite  
> helpful. I hope that Roberto can verify (or correct) this.
>
> I'm copying some folks who have been working with r.walk recently.  
> Our lab discussions on this caused me to raise these questions.
>
> On the issue I first asked about, it seems that lambda should either  
> 1) have a default value of 1 or 2) be optional.
>
> Michael
>
>
> On Oct 10, 2008, at 6:09 PM, Helena Mitasova wrote:
>
>>
>> On Oct 10, 2008, at 7:39 PM, Michael Barton wrote:
>>
>>>
>>>
>>>
>>> On Oct 10, 2008, at 11:32 AM, Helena Mitasova wrote:
>>>
>>>> I opened the code and it has it right in header:
>>>>
>>>> TOTAL COST = [(WALKING ENERGY ) + (LAMBDA*FRICTION)]
>>>>
>>>> maybe this is how it should go into the man page
>>>
>>> That seems like a good idea.
>>>
>>> This suggests that friction and lambda should be in some kind of  
>>> energy units.
>>>
>>> However, as I understand it, the values in an r.walk map--using  
>>> the default values--are an estimate of the number of seconds to  
>>> traverse a cell walking 'normally' (i.e., according to the default  
>>> values). Is this true anyone? If so, wouldn't the additive  
>>> friction need to be in time units?
>>>
>>> I'm not trying to be dense, but trying to get clear about what the  
>>> output is actually telling us, since it does not seem to be in  
>>> arbitrary units like r.cost is (unless you do some numerical  
>>> massaging).
>>
>> these are perfectly valid questions - authors should probably  
>> answer them rather than me
>> but we tried to put some explanation based on the manual in to the  
>> appendix of GRASSbook
>> and I have just covered it in the class so I had to spend some time  
>> trying to understand it.
>>
>> the units are - according to the manual - time - see below
>>
>> S, H are meters but the coefficients a,b,c,d are 1/speed which is  
>> sec/meter  giving you time in seconds,
>> then friction map can be either in units of time (sec) and lambda  
>> is unitless weight
>> or friction is unitless factor and lambda is in seconds which  
>> converts it to time.
>> So the results are in seconds - when you derive contours from the  
>> results you will get isochrones -
>> so you can delineate an area where the person gets within 2 hours  
>> or whatever time you chose.
>>
>> But it would be really good to hear from the authors because these  
>> are my interpretations
>> of the manual and my experiments with the module. The man page is  
>> pretty good it just needs to be more clear that the cost is  
>> measured by time (if I understand it correctly)
>>
>> Helena
>>
>>
>> T= [(a)*(Delta S)] + [(b)*(Delta H uphill)] + [(c)*(Delta H  
>> moderate downhill)] + [(d)*(Delta H steep downhill)]
>>
>> where:
>> T is time of movement in seconds,
>> Delta S is the distance covered in meters,
>> Delta H is the altitude difference in meter.
>>
>> The a, b, c, d parameters take in account movement speed in the  
>> different conditions and are linked to:
>>
>>   * a: underfoot condition (a=1/walking_speed)
>>   * b: underfoot condition and cost associated to movement uphill
>>   * c: underfoot condition and cost associated to movement moderate  
>> downhill
>>   * d: underfoot condition and cost associated to movement steep  
>> downhill
>>
>> It has been proved that moving downhill is favourable up to a  
>> specific slope value threshold, after that it becomes unfavourable.  
>> The default slope value threshold (slope factor) is -0.2125,  
>> corresponding to tan(-12), calibrated on human behaviour (>5 and  
>> <12 degrees: moderate downhill; >12 degrees: steep downhill). The  
>> default values for a, b, c, d are those proposed by Langmuir (0.72,  
>> 6.0, 1.9998, -1.9998), based on man walking effort in standard  
>> conditions.
>>
>> The lambda parameter of the linear equation combining movement and  
>> friction costs:
>> total cost = movement time cost + (lambda) * friction costs
>> must be set in the option section of r.walk.
>>
>>
>>>
>>>
>>> Michael
>>>
>>>
>>>
>>>>
>>>>
>>>>
>>>> On Oct 10, 2008, at 2:21 PM, Michael Barton wrote:
>>>>
>>>>> I've been emailing with Helena to try to understand exactly how  
>>>>> lambda and a friction surface interacts with information about  
>>>>> topography (extracted from a DEM) in r.walk. It seems a good  
>>>>> idea to put this back on the list. Perhaps I'm the only one a  
>>>>> little in the dark, but maybe it can help others.
>>>>>
>>>>> On Oct 10, 2008, at 10:40 AM, Helena Mitasova wrote:
>>>>>
>>>>>>>
>>>>>>> To clarify for me, is it
>>>>>>>
>>>>>>> total cost = (movement time cost + lambda) * friction costs
>>>>>>>
>>>>>>> OR
>>>>>>>
>>>>>>> total cost = movement time cost + (lambda * friction costs)
>>>>>>
>>>>>> I did not look into the code but if there are no brackets in  
>>>>>> the code, this second interpretation applies.
>>>>>
>>>>> For anyone familiar with the code, is this the case? If so,  
>>>>> should I be thinking in time units for creating a friction map?  
>>>>> If I remember, r.walk normally outputs in seconds to traverse  
>>>>> the cell.
>>>>>
>>>>> So, should the friction map be in additional seconds to traverse  
>>>>> the cell? Or is friction a weighting factor (i.e.,  
>>>>> multiplicative rather than additive)?
>>>>>
>>>>> Thanks
>>>>> Michael
>>>>>
>>>>
>>>
>>
>

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