NAME¶
r.los - Line-of-sight raster analysis program.
KEYWORDS¶
raster, viewshed
SYNOPSIS¶
r.los
r.los help
r.los [-
c]
input=
name output=
name
coordinate=
x,y [
patt_map=
name]
[
obs_elev=
float] [
max_dist=
float]
[--
overwrite] [--
verbose] [--
quiet]
Flags:¶
- -c
-
Consider earth curvature (current ellipsoid)
- --overwrite
-
Allow output files to overwrite existing files
- --verbose
-
Verbose module output
- --quiet
-
Quiet module output
Parameters:¶
- input=name
-
Name of elevation raster map
- output=name
-
Name for output raster map
- coordinate=x,y
-
Coordinate identifying the viewing position
- patt_map=name
-
Binary (1/0) raster map to use as a mask
- obs_elev=float
-
Viewing position height above the ground
Default: 1.75
- max_dist=float
-
Maximum distance from the viewing point (meters)
Options: 0-5000000
Default: 10000
DESCRIPTION¶
r.los generates a raster output map in which the cells that are visible
from a user-specified observer position are marked with the vertical angle (in
degrees) required to see those cells (viewshed). A value of 0 is directly
below the specified viewing position, 90 is due horizontal, and 180 is
directly above the observer. The angle to the cell containing the viewing
position is undefined and set to 180.
To run
r.los, the user must specify at least an
input map name,
output map name, and the geographic
coordinates of the user's
viewing location; any remaining parameters whose values are unspecified will
be set to their default values (see below).
The
patt_map is the name of a binary (1/0) raster map layer in which
cells within the areas of interest are assigned the category value '1', and
all other cells are assigned the category value '0' or NULL. If this parameter
is omitted, the analysis will be performed for the whole area within a certain
distance of the viewing point inside the geographic region boundaries.
Default: assign all cells that are within the
max_dist and within the
user's current geographic region boundaries a value of 1.
The
obs_elev parameter defines the height of the observer (in meters)
above the viewing point's elevation.
The
max_dist parameter is the maximum distance (in meters) from the
viewing point inside of which the line of sight analysis will be performed.
The cells outside this distance range are assigned a NULL value.
NOTES¶
For accurate results, the program must be run with the resolution of the
geographic region set equal to the resolution of the data (see
g.region).
The time to complete the calculation increases dramatically with the region
size. Try to keep the columns and rows under 1000.
It is advisable to use a 'pattern layer' which identifies the areas of interest
in which the line of sight analysis is required. Such a measure will reduce
the time taken by the program to run.
The curvature of the Earth is not taken into account for these calculations.
However, for interest's sake, a handy calculation for distance to the true
horizon is approximated by
d = sqrt(13*h) where
h is the height
of the observer in meters (above sea level) and
d is the distance to
the horizon in km. This may be useful for setting the
max_dist value.
EXAMPLE¶
Spearfish example - calculation of viewshed from 50m tower on top of a mountain:
g.region rast=elevation.dem -p
r.los elevation.dem out=los coord=598869,4916642 obs_elev=50 max_dist=10000
r.colors -e los color=bgyr
d.shadedmap relief=aspect drape=los bright=10
echo "symbol extra/target 25 598869 4916642 red" | d.graph -m
TODO¶
a) Rewrite using ideas from
r.cva and a method which scales better to
large regions.
A suggested method is detailed in: Izraelevitz, David (USACE).
’A Fast Algorithm for Approximate Viewshed Computation'
Photogrammetric Engineering & Remote Sensing, July 2003 b) or fix
r.viewshed in Addons
SEE ALSO¶
g.region
AUTHOR¶
Kewan Q. Khawaja, Intelligent Engineering Systems Laboratory, M.I.T.
Last changed: $Date: 2008-12-14 12:25:21 +0100 (Sun, 14 Dec 2008) $
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