1r.walk(1)                     Grass User's Manual                    r.walk(1)
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3
4

NAME

6       r.walk  - Outputs a raster map layer showing the anisotropic cumulative
7       cost of moving between different geographic locations on an input  ele‐
8       vation  raster map layer whose cell category values represent elevation
9       combined with an input raster map layer  whose  cell  values  represent
10       friction cost.
11

KEYWORDS

13       raster
14

SYNOPSIS

16       r.walk
17       r.walk help
18       r.walk    [-vknr]    elevation=string   friction=string   output=string
19       [start_points=string]           [stop_points=string]           [coordi‐
20       nate=x,y[,x,y,...]]   [stop_coordinate=x,y[,x,y,...]]   [max_cost=cost]
21       [null_cost=null cost]   [percent_memory=percent  memory]    [nseg=nseg]
22       [walk_coeff=a,b,c,d]      lambda=lambda     [slope_factor=slope_factor]
23       [--overwrite]
24
25   Flags:
26       -v  Run verbosely
27
28       -k  Use the 'Knight's move'; slower, but more accurate
29
30       -n  Keep null values in output map
31
32       -r  Start with values in raster map
33
34       --overwrite
35
36   Parameters:
37       elevation=string
38           Name of elevation input raster map
39
40       friction=string
41           Name of input raster map containing friction costs
42
43       output=string
44           Name of raster map to contain results
45
46       start_points=string
47           Starting points vector map
48
49       stop_points=string
50           Stop points vector map
51
52       coordinate=x,y[,x,y,...]
53           The map E and N grid coordinates of a starting point (E,N)
54
55       stop_coordinate=x,y[,x,y,...]
56           The map E and N grid coordinates of a stopping point (E,N)
57
58       max_cost=cost
59           An optional maximum cumulative cost Default: 0
60
61       null_cost=null cost
62           Cost assigned to null cells. By default, null cells are excluded
63
64       percent_memory=percent memory
65           Percent of map to keep in memory Default: 100
66
67       nseg=nseg
68           Number of the segment to create (segment library) Default: 4
69
70       walk_coeff=a,b,c,d
71           Coefficients for walking energy formula parameters a,b,c,d Default:
72           0.72,6.0,1.9998,-1.9998
73
74       lambda=lambda
75           Lambda coefficients for combining walking energy and friction cost
76
77       slope_factor=slope_factor
78           Slope factor determines travel energy cost per height step Default:
79           -0.2125
80

DESCRIPTION

82       r.walk outputs a raster map layer showing the lowest cumulative cost of
83       moving  between  each  cell  and the user-specified starting points. It
84       uses an input elevation raster map layer  whose  cell  category  values
85       represent  elevation,  combined  with  a  second input raster map layer
86       whose cell values represent friction costs.  This function  is  similar
87       to  r.cost,  but in addiction to a friction map, it considers an aniso‐
88       tropic travel time due to the different walking speed  associated  with
89       downhill and uphill movements.
90
91       The  formula  from  Aitken 1977/Langmuir 1984 (based on Naismith's rule
92       for walking times) has been used to estimate  the  cost  parameters  of
93       specific slope intervals:
94
95       T=  [(a)*(Delta  S)]  + [(b)*(Delta H uphill)] + [(c)*(Delta H moderate
96       downhill)] + [(d)*(Delta H steep downhill)]
97
98       where:
99       T is time of movement in seconds,
100       Delta S is the distance covered in meters,
101       Delta H is the altitude difference in meter.
102
103       The a, b, c, d parameters take in account movement speed in the differ‐
104       ent conditions and are linked to:
105
106                     a: underfoot condition (a=1/walking_speed)
107
108                     b:  underfoot  condition  and cost associated to movement
109                     uphill
110
111                     c: underfoot condition and cost  associated  to  movement
112                     moderate downhill
113
114                     d:  underfoot  condition  and cost associated to movement
115                     steep downhill
116       It has been proved that moving downhill is favourable up to a  specific
117       slope  value threshold, after that it becomes unfavourable. The default
118       slope value threshold  (slope  factor)  is  -0.2125,  corresponding  to
119       tan(-12),  calibrated  on human behaviour (>5 and <12 degrees: moderate
120       downhill; >12 degrees: steep downhill). The default values for a, b, c,
121       d are those proposed by Langmuir (0.72, 6.0, 1.9998, -1.9998), based on
122       man walking effort in standard conditions.
123
124       The lambda parameter of the  linear  equation  combining  movement  and
125       friction costs:
126       total cost = movement time cost + (lambda) * friction costs
127       must be set in the option section of r.walk.
128
129       For  a  more  accurate  result,  the "knight's move" option can be used
130       (although it is more time consuming). In the diagram below, the  center
131       location (O) represents a grid cell from which cumulative distances are
132       calculated. Those neighbours marked with an x are always considered for
133       cumulative  cost  updates.  With the "knight's move" option, the neigh‐
134       bours marked with a K are also considered.
135         K   K
136       K x x x K
137         x O x
138       K x x x K
139         K   K
140
141
142       The minimum cumulative costs are computed using  Dijkstra's  algorithm,
143       that  find  an optimum solution (for more details see r.cost, that uses
144       the same algorithm).
145
146       Once r.walk computes the cumulative cost map as a linear combination of
147       friction cost (from friction map) and the altitude and distance covered
148       (from the digital elevation model), r.drain can be  used  to  find  the
149       minimum cost path.
150

SEE ALSO

152       r.cost, r.drain, r.in.ascii, r.mapcalc, r.out.ascii
153

REFERENCES

155                     Aitken,  R.  1977.  Wilderness  areas in Scotland. Unpub‐
156                     lished Ph.D. thesis.  University of Aberdeen.
157
158                      Steno Fontanari, University of Trento, Italy, Ingegneria
159                     per  l'Ambiente e il Territorio, 2000-2001.  Svilluppo di
160                     metodologie  GIS  per  la  determinazione   dell'accessi‐
161                     bilità  territoriale  come supporto alle decisioni
162                     nella gestione ambientale.
163
164                     Langmuir, E.  1984.  Mountaincraft  and  leadership.  The
165                     Scottish Sports Council/MLTB. Cordee, Leicester.
166

AUTHORS

168       Based on r.cost written by :
169
170       Antony Awaida,
171       Intelligent Engineering
172       Systems Laboratory,
173       M.I.T.
174       James Westervelt,
175       U.S.Army Construction Engineering Research Laboratory
176
177       Updated for Grass 5
178       Pierre de Mouveaux (pmx@audiovu.com)
179
180       Initial version of r.walk:
181
182       Steno Fontanari, 2002
183
184       Current version of r.walk:
185
186       Franceschetti  Simone, Sorrentino Diego, Mussi Fabiano and Pasolli Mat‐
187       tia
188       Correction by: Fontanari Steno, Napolitano Maurizio and  Flor Roberto
189       In collaboration with: Franchi Matteo, Vaglia Beatrice, Bartucca Luisa,
190       Fava Valentina and Tolotti Mathias, 2004
191
192       Updated for Grass 6.1
193
194       Roberto Flor and Markus Neteler
195
196       Last changed: $Date: 2005/12/16 21:04:29 $
197
198       Full index
199
200
201
202GRASS 6.2.2                                                          r.walk(1)