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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       © 2003-2013 GRASS Development Team

GRASS 6.4.3                                                                                        r.los(1grass)