Provided by: grass-doc_8.4.1-1_all 
NAME
r.spread - Simulates elliptically anisotropic spread.
Generates a raster map of the cumulative time of spread, given raster maps containing the rates of spread
(ROS), the ROS directions and the spread origins. It optionally produces raster maps to contain backlink
UTM coordinates for tracing spread paths. Usable for fire spread simulations.
KEYWORDS
raster, fire, spread, hazard, model
SYNOPSIS
r.spread
r.spread --help
r.spread [-si] base_ros=string max_ros=string direction_ros=string start=string
[spotting_distance=string] [wind_speed=string] [fuel_moisture=string] [least_size=odd int]
[comp_dens=decimal] [init_time=int (>= 0)] [lag=int (>= 0)] [backdrop=string] output=string
[x_output=string] [y_output=string] [--overwrite] [--help] [--verbose] [--quiet] [--ui]
Flags:
-s
Consider spotting effect (for wildfires)
-i
Use start raster map values in output spread time raster map
Designed to be used with output of previous run of r.spread when computing spread iteratively. The
values in start raster map are considered as time. Allowed values in raster map are from zero to the
value of init_time option. If not enabled, init_time is used in the area of start raster map
--overwrite
Allow output files to overwrite existing files
--help
Print usage summary
--verbose
Verbose module output
--quiet
Quiet module output
--ui
Force launching GUI dialog
Parameters:
base_ros=string [required]
Raster map containing base ROS (cm/min)
Name of an existing raster map layer in the user’s current mapset search path containing the ROS
values in the directions perpendicular to maximum ROSes’ (cm/minute). These ROSes are also the ones
without the effect of directional factors.
max_ros=string [required]
Raster map containing maximal ROS (cm/min)
Name of an existing raster map layer in the user’s current mapset search path containing the maximum
ROS values (cm/minute).
direction_ros=string [required]
Raster map containing directions of maximal ROS (degree)
Name of an existing raster map layer in the user’s current mapset search path containing directions
of the maximum ROSes, clockwise from north (degree).
start=string [required]
Raster map containing starting sources
Name of an existing raster map layer in the user’s current mapset search path containing starting
locations of the spread phenomenon. Any positive integers in this map are recognized as starting
sources (seeds).
spotting_distance=string
Raster map containing maximal spotting distance (m, required with -s)
Name of an existing raster map layer in the user’s current mapset search path containing the maximum
potential spotting distances (meters).
wind_speed=string
Raster map containing midflame wind speed (ft/min, required with -s)
Name of an existing raster map layer in the user’s current mapset search path containing wind
velocities at half of the average flame height (feet/minute).
fuel_moisture=string
Raster map containing fine fuel moisture of the cell receiving a spotting firebrand (%, required with
-s)
Name of an existing raster map layer in the user’s current mapset search path containing the 1-hour
(<.25") fuel moisture (percentage content multiplied by 100).
least_size=odd int
Basic sampling window size needed to meet certain accuracy (3)
An odd integer ranging 3 - 15 indicating the basic sampling window size within which all cells will
be considered to see whether they will be reached by the current spread cell. The default number is 3
which means a 3x3 window.
Options: 3, 5, 7, 9, 11, 13, 15
comp_dens=decimal
Sampling density for additional computing (range: 0.0 - 1.0 (0.5))
A decimal number ranging 0.0 - 1.0 indicating additional sampling cells will be considered to see
whether they will be reached by the current spread cell. The closer to 1.0 the decimal number is, the
longer the program will run and the higher the simulation accuracy will be. The default number is
0.5.
init_time=int (>= 0)
Initial time for current simulation (0) (min)
A non-negative number specifying the initial time for the current spread simulation (minutes). This
is useful when multiple phase simulation is conducted. The default time is 0.
Default: 0
lag=int (>= 0)
Simulating time duration LAG (fill the region) (min)
A non-negative integer specifying the simulating duration time lag (minutes). The default is
infinite, but the program will terminate when the current geographic region/mask has been filled. It
also controls the computational time, the shorter the time lag, the faster the program will run.
backdrop=string
Name of raster map as a display backdrop
Name of an existing raster map layer in the user’s current mapset search path to be used as the
background on which the "live" movement will be shown.
output=string [required]
Raster map to contain output spread time (min)
Name of the new raster map layer to contain the results of the cumulative spread time needed for a
phenomenon to reach each cell from the starting sources (minutes).
x_output=string
Name of raster map to contain X back coordinates
Name of the new raster map layer to contain the results of backlink information in UTM easting
coordinates for each cell.
y_output=string
Name of raster map to contain Y back coordinates
Name of the new raster map layer to contain the results of backlink information in UTM northing
coordinates for each cell.
DESCRIPTION
r.spread is part of the wildfire simulation toolset. Preparational steps for the fire simulation are the
calculation of the rate of spread (ROS) with r.ros, and the creating of spread map with r.spread.
Eventually, the fire path(s) based on starting point(s) are calculated with r.spreadpath.
Spread phenomena usually show uneven movement over space. Such unevenness is due to two reasons:
1 the uneven conditions from location to location, which can be called spatial heterogeneity, and
2 the uneven conditions in different directions, which can be called anisotropy.
The anisotropy of spread occurs when any of the determining factors have directional components. For
example, wind and topography cause anisotropic spread of wildfires.
One of the simplest spatial heterogeneous and anisotropic spread is elliptical spread, in which, each
local spread shape can be thought as an ellipse. In a raster setting, cell centers are foci of the spread
ellipses, and the spread phenomenon moves fastest toward apogees and slowest to perigees. The sizes and
shapes of spread ellipses may vary cell by cell. So the overall spread shape is commonly not an ellipse.
r.spreadsimulates elliptically anisotropic spread phenomena, given three raster map layers about ROS
(base ROS, maximum ROS and direction of the maximum ROS) plus a raster map layer showing the starting
sources. These ROS layers define unique ellipses for all cell locations in the current computational
region as if each cell center was a potential spread origin. For some wildfire spread, these ROS layers
can be generated by another GRASS raster program r.ros. The actual locations reached by a spread event
are constrained by the actual spread origins and the elapsed spread time.
r.spreadoptionally produces raster maps to contain backlink UTM coordinates for each raster cell of the
spread time map. The spread paths can be accurately traced based on the backlink information by
r.spreadpath module.
Part of the spotting function in r.spread is based on Chase (1984) and Rothermel (1983). More information
on r.spread, r.ros and r.spreadpath can be found in Xu (1994).
Options spot_dist, w_speed and f_mois must all be given if the -s (spotting) flag is used.
EXAMPLE
Assume we have inputs, the following simulates a spotting- involved wildfire and generates three raster
maps to contain spread time, backlink information in UTM northing and easting coordinates:
r.spread -s max=my_ros.max dir=my_ros.maxdir base=my_ros.base \
start=fire_origin spot_dist=my_ros.spotdist w_speed=wind_speed \
f_mois=1hour_moisture output=my_spread \
x_output=my_spread.x y_output=my_spread.y
NOTES
1. r.spread is a specific implementation of the shortest path algorithm. r.cost module served as the
starting point for the development of r.spread. One of the major differences between the two programs is
that r.cost only simulates isotropic spread while r.spread can simulate elliptically anisotropic spread,
including isotropic spread as a special case.
2. Before running r.spread, the user should prepare the ROS (base, max and direction) maps using
appropriate models. For some wildfire spread, the r.ros module based on Rothermel’s fire equation does
such work. The combination of the two forms a simulation of wildfire spread.
3. The relationship of the start map and ROS maps should be logically correct, i.e. a starting source (a
positive value in the start map) should not be located in a spread barrier (zero value in the ROS maps).
Otherwise the program refuses to run.
4. r.spread uses the current computational region settings. The output map layer will not go outside the
boundaries set in the region, and will not be influenced by starting sources outside. So any change of
the current region may influence the output. The recommendation is to use slightly larger region than
needed. Refer to g.region to set an appropriate computational region.
5. The user should be sure that the inputs to r.spread are in proper units.
6. r.spread is a computationally intensive program. The user may need to choose appropriate size of the
computational region and resolution.
7. A low and medium (i.e. <= 0.5) sampling density can improve accuracy for elliptical simulation
significantly, without adding significantly extra running time. Further increasing the sample density
will not gain much accuracy while requiring greatly additional running time.
REFERENCES
• Chase, Carolyn, H., 1984, Spotting distance from wind-driven surface fires -- extensions of
equations for pocket calculators, US Forest Service, Res. Note INT-346, Ogden, Utah.
• Rothermel, R. C., 1983, How to predict the spread and intensity of forest and range fires. US
Forest Service, Gen. Tech. Rep. INT-143. Ogden, Utah.
• Xu, Jianping, 1994, Simulating the spread of wildfires using a geographic information system and
remote sensing, Ph. D. Dissertation, Rutgers University, New Brunswick, New Jersey (ref).
SEE ALSO
r.cost, r.mask, r.ros, r.spreadpath Sample data download: firedemo.sh (run this script within the "Fire
simulation data set" project).
AUTHORS
Jianping Xu and Richard G. Lathrop, Jr., Center for Remote Sensing and Spatial Analysis, Rutgers
University.
SOURCE CODE
Available at: r.spread source code (history)
Accessed: Friday Apr 04 01:19:34 2025
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GRASS 8.4.1 r.spread(1grass)