Provided by: grass-doc_6.4.3-3_all 
NAME
r.walk - Outputs a raster map layer showing the anisotropic cumulative cost of moving
between different geographic locations on an input elevation raster map layer whose cell
category values represent elevation combined with an input raster map layer whose cell
values represent friction cost.
KEYWORDS
raster, cost surface, cumulative costs
SYNOPSIS
r.walk
r.walk help
r.walk [-knr] elevation=string friction=string output=string [outdir=string]
[start_points=string] [stop_points=string] [coordinate=x,y[,x,y,...]]
[stop_coordinate=x,y[,x,y,...]] [max_cost=integer] [null_cost=float]
[percent_memory=integer] [nseg=integer] [walk_coeff=a,b,c,d] [lambda=float]
[slope_factor=float] [--overwrite] [--verbose] [--quiet]
Flags:
-k
Use the 'Knight's move'; slower, but more accurate
-n
Keep null values in output map
-r
Start with values in raster map
--overwrite
Allow output files to overwrite existing files
--verbose
Verbose module output
--quiet
Quiet module output
Parameters:
elevation=string
Name of elevation input raster map
friction=string
Name of input raster map containing friction costs
output=string
Name of raster map to contain results
outdir=string
Name of output raster map to contain movement directions
start_points=string
Starting points vector map
stop_points=string
Stop points vector map
coordinate=x,y[,x,y,...]
The map E and N grid coordinates of a starting point (E,N)
stop_coordinate=x,y[,x,y,...]
The map E and N grid coordinates of a stopping point (E,N)
max_cost=integer
An optional maximum cumulative cost
Default: 0
null_cost=float
Cost assigned to null cells. By default, null cells are excluded
percent_memory=integer
Percent of map to keep in memory
Default: 100
nseg=integer
Number of the segment to create (segment library)
Default: 4
walk_coeff=a,b,c,d
Coefficients for walking energy formula parameters a,b,c,d
Default: 0.72,6.0,1.9998,-1.9998
lambda=float
Lambda coefficients for combining walking energy and friction cost
Default: 1.0
slope_factor=float
Slope factor determines travel energy cost per height step
Default: -0.2125
DESCRIPTION
r.walk outputs 1) a raster map layer showing the lowest cumulative cost of moving between
each cell and the user-specified starting points and 2) a second raster map layer showing
the movement direction to the next cell on the path back to the start point (see Movement
Direction). It uses an input elevation raster map layer whose cell category values
represent elevation, combined with a second input raster map layer whose cell values
represent friction costs. This function is similar to r.cost, but in addiction to a
friction map, it considers an anisotropic travel time due to the different walking speed
associated with downhill and uphill movements.
The formula from Aitken 1977/Langmuir 1984 (based on Naismith's rule for walking times)
has been used to estimate the cost parameters of specific slope intervals:
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.
For a more accurate result, the "knight's move" option can be used (although it is more
time consuming). In the diagram below, the center location (O) represents a grid cell from
which cumulative distances are calculated. Those neighbours marked with an x are always
considered for cumulative cost updates. With the "knight's move" option, the neighbours
marked with a K are also considered.
K K
K x x x K
x O x
K x x x K
K K
The minimum cumulative costs are computed using Dijkstra's algorithm, that find an optimum
solution (for more details see r.cost, that uses the same algorithm).
Movement Direction
The movement direction surface is created to record the sequence of movements that created
the cost accumulation surface. Without it r.drain would not correctly create a path from
an end point back to the start point. The direction shown in each cell points away from
the cell that came before it. The directions are recorded as
112.5 90 67.5 i.e. a cell with the value 135
157.5 135 0 45 22.5 means the cell before it is
180 x 0 to the south-east.
202.5 225 270 315 337.5
247.5 292.5
Once r.walk computes the cumulative cost map as a linear combination of friction cost
(from friction map) and the altitude and distance covered (from the digital elevation
model), r.drain can be used to find the minimum cost path. Make sure to use the -d flag
and the movement direction raster map when running r.drain to ensure the path is computed
according to the proper movement directions.
SEE ALSO
r.cost, r.drain, r.in.ascii, r.mapcalc, r.out.ascii
REFERENCES
Aitken, R. 1977. Wilderness areas in Scotland. Unpublished Ph.D. thesis.
University of Aberdeen.
Steno Fontanari, University of Trento, Italy, Ingegneria per l'Ambiente e
il Territorio, 2000-2001. Svilluppo di metodologie GIS per la
determinazione dell'accessibilità territoriale come supporto alle
decisioni nella gestione ambientale.
Langmuir, E. 1984. Mountaincraft and leadership. The Scottish Sports
Council/MLTB. Cordee, Leicester.
AUTHORS
Based on r.cost written by :
Antony Awaida,
Intelligent Engineering
Systems Laboratory,
M.I.T.
James Westervelt,
U.S.Army Construction Engineering Research Laboratory
Updated for Grass 5
Pierre de Mouveaux (pmx@audiovu.com)
Initial version of r.walk:
Steno Fontanari, 2002
Current version of r.walk:
Franceschetti Simone, Sorrentino Diego, Mussi Fabiano and Pasolli Mattia
Correction by: Fontanari Steno, Napolitano Maurizio and Flor Roberto
In collaboration with: Franchi Matteo, Vaglia Beatrice, Bartucca Luisa, Fava Valentina and
Tolotti Mathias, 2004
Updated for Grass 6.1
Roberto Flor and Markus Neteler
Last changed: $Date: 2012-12-31 04:29:35 -0800 (Mon, 31 Dec 2012) $
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