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 03:25:21 -0800 (Sun, 14 Dec 2008) $

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