1r.walk(1) Grass User's Manual r.walk(1)
2
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
13 raster
14
16 r.walk
17 r.walk help
18 r.walk [-knr] 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] [--verbose] [--quiet]
24 Flags:
26 -k
27 Use the 'Knight's move'; slower, but more accurate
28 -n
30 Keep null values in output map
31 -r
33 Start with values in raster map
34 --overwrite
36 Allow output files to overwrite existing files
37 --verbose
39 Verbose module output
40 --quiet
42 Quiet module output
43 Parameters:
45 elevation=string
46 Name of elevation input raster map
47 friction=string
49 Name of input raster map containing friction costs
50 output=string
52 Name of raster map to contain results
53 start_points=string
55 Starting points vector map
56 stop_points=string
58 Stop points vector map
59 coordinate=x,y[,x,y,...]
61 The map E and N grid coordinates of a starting point (E,N)
62 stop_coordinate=x,y[,x,y,...]
64 The map E and N grid coordinates of a stopping point (E,N)
65 max_cost=cost
67 An optional maximum cumulative cost
68 Default: 0
69 null_cost=null cost
71 Cost assigned to null cells. By default, null cells are excluded
72 percent_memory=percent memory
74 Percent of map to keep in memory
75 Default: 100
76 nseg=nseg
78 Number of the segment to create (segment library)
79 Default: 4
80 walk_coeff=a,b,c,d
82 Coefficients for walking energy formula parameters a,b,c,d
83 Default: 0.72,6.0,1.9998,-1.9998
84 lambda=lambda
86 Lambda coefficients for combining walking energy and friction cost
87 slope_factor=slope_factor
89 Slope factor determines travel energy cost per height step
90 Default: -0.2125
91
93 r.walk outputs a raster map layer showing the lowest cumulative cost of
94 moving between each cell and the user-specified starting points. It
95 uses an input elevation raster map layer whose cell category values
96 represent elevation, combined with a second input raster map layer
97 whose cell values represent friction costs. This function is similar
98 to r.cost, but in addiction to a friction map, it considers an aniso‐
99 tropic travel time due to the different walking speed associated with
100 downhill and uphill movements.
101 The formula from Aitken 1977/Langmuir 1984 (based on Naismith's rule
103 for walking times) has been used to estimate the cost parameters of
104 specific slope intervals:
105 T= [(a)*(Delta S)] + [(b)*(Delta H uphill)] + [(c)*(Delta H moderate
107 downhill)] + [(d)*(Delta H steep downhill)]
108 where:
110 T is time of movement in seconds,
111 Delta S is the distance covered in meters,
112 Delta H is the altitude difference in meter.
113 The a, b, c, d parameters take in account movement speed in the differ‐
115 ent conditions and are linked to:
116 a: underfoot condition (a=1/walking_speed)
118 b: underfoot condition and cost associated to movement
120 uphill
121 c: underfoot condition and cost associated to movement
123 moderate downhill
124 d: underfoot condition and cost associated to movement
126 steep downhill
127 It has been proved that moving downhill is favourable up to a specific
128 slope value threshold, after that it becomes unfavourable. The default
129 slope value threshold (slope factor) is -0.2125, corresponding to
130 tan(-12), calibrated on human behaviour (>5 and <12 degrees: moderate
131 downhill; >12 degrees: steep downhill). The default values for a, b, c,
132 d are those proposed by Langmuir (0.72, 6.0, 1.9998, -1.9998), based on
133 man walking effort in standard conditions.
134 The lambda parameter of the linear equation combining movement and
136 friction costs:
137 total cost = movement time cost + (lambda) * friction costs
138 must be set in the option section of r.walk.
139 For a more accurate result, the "knight's move" option can be used
141 (although it is more time consuming). In the diagram below, the center
142 location (O) represents a grid cell from which cumulative distances are
143 calculated. Those neighbours marked with an x are always considered for
144 cumulative cost updates. With the "knight's move" option, the neigh‐
145 bours marked with a K are also considered.
146 K K
147 K x x x K
148 x O x
149 K x x x K
150 K K
151 The minimum cumulative costs are computed using Dijkstra's algorithm,
154 that find an optimum solution (for more details see r.cost, that uses
155 the same algorithm).
156 Once r.walk computes the cumulative cost map as a linear combination of
158 friction cost (from friction map) and the altitude and distance covered
159 (from the digital elevation model), r.drain can be used to find the
160 minimum cost path.
161
163 r.cost, r.drain, r.in.ascii, r.mapcalc, r.out.ascii
164
166 Aitken, R. 1977. Wilderness areas in Scotland. Unpub‐
167 lished Ph.D. thesis. University of Aberdeen.
168 Steno Fontanari, University of Trento, Italy, Ingegneria
170 per l'Ambiente e il Territorio, 2000-2001. Svilluppo di
171 metodologie GIS per la determinazione dell'accessi‐
172 bilità territoriale come supporto alle decisioni
173 nella gestione ambientale.
174 Langmuir, E. 1984. Mountaincraft and leadership. The
176 Scottish Sports Council/MLTB. Cordee, Leicester.
177
179 Based on r.cost written by :
180 Antony Awaida,
182 Intelligent Engineering
183 Systems Laboratory,
184 M.I.T.
185 James Westervelt,
186 U.S.Army Construction Engineering Research Laboratory
187 Updated for Grass 5
189 Pierre de Mouveaux (pmx@audiovu.com)
190 Initial version of r.walk:
192 Steno Fontanari, 2002
194 Current version of r.walk:
196 Franceschetti Simone, Sorrentino Diego, Mussi Fabiano and Pasolli Mat‐
198 tia
199 Correction by: Fontanari Steno, Napolitano Maurizio and Flor Roberto
200 In collaboration with: Franchi Matteo, Vaglia Beatrice, Bartucca Luisa,
201 Fava Valentina and Tolotti Mathias, 2004
202 Updated for Grass 6.1
204 Roberto Flor and Markus Neteler
206 Last changed: $Date: 2005-12-16 22:04:29 +0100 (Fri, 16 Dec 2005) $
208 Full index
210 © 2003-2008 GRASS Development Team
212GRASS 6.3.0 r.walk(1)