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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