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NAME

       r.viewshed  - Computes the viewshed of a point on an elevation raster map.
       Default format: NULL (invisible), vertical angle wrt viewpoint (visible).

KEYWORDS

       raster, viewshed, line of sight, LOS

SYNOPSIS

       r.viewshed
       r.viewshed --help
       r.viewshed        [-crbe]        input=name       output=name       coordinates=east,north
       [observer_elevation=value]          [target_elevation=value]          [max_distance=value]
       [refraction_coeff=float]    [memory=value]    [directory=string]   [--overwrite]  [--help]
       [--verbose]  [--quiet]  [--ui]

   Flags:
       -c
           Consider the curvature of the earth (current ellipsoid)

       -r
           Consider the effect of atmospheric refraction

       -b
           Output format is invisible = 0, visible = 1

       -e
           Output format is invisible = NULL, else current elev - viewpoint_elev

       --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:
       input=name [required]
           Name of input elevation raster map

       output=name [required]
           Name for output raster map

       coordinates=east,north [required]
           Coordinates of viewing position

       observer_elevation=value
           Viewing elevation above the ground
           Default: 1.75

       target_elevation=value
           Offset for target elevation above the ground
           Default: 0.0

       max_distance=value
           Maximum visibility radius. By default infinity (-1)
           Default: -1

       refraction_coeff=float
           Refraction coefficient
           Options: 0.0-1.0
           Default: 0.14286

       memory=value
           Amount of memory to use in MB
           Default: 500

       directory=string
           Directory to hold temporary files (they can be large)

DESCRIPTION

       r.viewshed is a module that computes the viewshed of a point on a raster terrain. That is,
       given  an  elevation raster, and the location of an observer, it generates a raster output
       map showing which cells are visible from the given  location.   The  algorithm  underlying
       r.viewshed  minimizes both the CPU operations and the transfer of data between main memory
       and disk; as a result r.viewshed runs fast on very large rasters.

NOTES

       To run r.viewshed, the user must specify an input elevation map name, an output raster map
       name, and the location of the viewpoint.

       For  the  time being the viewpoint (coordinates parameter) is assumed to be located inside
       the terrain.  The viewpoint location is given in map coordinates.

       The output raster map may have one of three possible formats, based  on  which  flags  are
       set.

       By  default,  if no flag is set, the output is in angle-mode, and each point in the output
       map is marked as NULL if the point is not visible or the respective point in the elevation
       map  is  NULL.  Otherwise, a value in [0, 180] representing the vertical angle with regard
       to the viewpoint, in degrees, if the point is visible.  A value of 0 is directly below the
       specified  viewing  position,  90  is due horizontal. The angle to the cell containing the
       viewing position is undefined and set to 180.

       If the -b flag is set, the output is in boolean-mode, and each point in the output map  is
       marked as:

           ·   0 if the point is no-data/null or not visible

           ·   1 if the point is visible.

       If  the  -e flag is set, the output is in elevation-mode, and each point in the output map
       is marked as:

           ·   no-data (null), if the respective point in the elevation map is no-data (null)

           ·   -1, if the point is not visible

           ·   the difference in elevation between the point and the viewpoint, if the  point  is
               visible.

       If  you  wish  to  identify  the area of the map which is within the search radius but not
       visible, a combination of r.buffer and r.mapcalc can be used to create a negative  of  the
       viewshed map.

       By  default  the  elevations are not adjusted for the curvature of the earth. The user can
       turn this on with flag -c.

       By default the observer is assumed to have height 1.75 map units above the  terrain.   The
       user  can  change  this  using option observer_elevation. The value entered is in the same
       units as the elevation.

       By default the target is assumed to have height of 0 map units  above  the  terrain.   The
       user  can  change  this  using  option target_elevation to determine if objects of a given
       height would be visible. The value entered is in the same units as the elevation.

       By default there is no restriction on the maximum distance to which the observer can  see.
       The  user  can  set a maximum distance of visibility using option max_distance.  The value
       entered is in the same units as the cell size of the raster.

       Main memory usage: By default r.viewshed assumes it has 500MB of main memory, and sets  up
       its  internal  data  structures  so that it does not require more than this amount of RAM.
       The user can set the amount of memory used by the program by setting  the  memory  to  the
       number of MB of memory they would like to be used.

   Memory mode
       The  algorithm  can  run  in  two modes: in internal memory, which means that it keeps all
       necessary data structures in memory during the computation. And in external memory,  which
       means  that the data structures are external, i.e. on disk.  r.viewshed decides which mode
       to run in using the amount of main memory specified by the user.   The  internal  mode  is
       (much) faster than the external mode.

       Ideally, the user should specify on the command line the amount of physical memory that is
       free for the program to use. Underestimating the memory may result in  r.viewshed  running
       in  external  mode instead of internal, which is slower. Overestimating the amount of free
       memory may result in r.viewshed running in internal mode and using virtual  memory,  which
       is slower than the external mode.

   The algorithm
       r.viewshed  uses  the  following model for determining visibility: The height of a cell is
       assumed to be variable, and the actual height of a point falling  into  a  cell,  but  not
       identical  the  cell  center,  is  interpolated.  Thus  the  terrain is viewed as a smooth
       surface.  Two points are visible to each other if their line-of-sight does  not  intersect
       the terrain. The height for an arbitrary point x in the terrain is interpolated from the 4
       surrounding neighbours. This means that  this  model  does  a  bilinear  interpolation  of
       heights.   This  model  is  suitable  for  both low and high resolution rasters as well as
       terrain with flat and steep slopes.

       The core of the algorithm is  determining,  for  each  cell,  the  line-of-sight  and  its
       intersections  with the cells in the terrain. For a (square) grid of n cells, there can be
       O(n 1/2) cells that intersect the LOS. If we test every single such cell for  every  point
       in  the grid, this adds up to O(n3/2) tests. We can do all these tests faster if we re-use
       information from one point to the next (two grid points that are close to each other  will
       be intersected by a lot of the same points) and organize the computation differently.

       More  precisely,  the  algorithm  uses  a  technique  called line sweeping: It considers a
       half-line centered at the viewpoint, and rotates it radially  around  the  viewpoint,  360
       degrees.   During  the sweep it keeps track of all the cells that intersect the sweep line
       at that time; These are called the active cells. A cell has 3 associated events:  when  it
       is first met by the sweep line and inserted into the active structure; when it is last met
       by the sweep line and deleted from the active structure; and when the  sweep  line  passes
       over  its  centerpoint,  at  which  time  its  visibility is determined.  To determine the
       visibility of a cell all cells that intersect the line-of-sight must be  active,  so  they
       are in the active structure.  The algorithm looks at all the active cells that are between
       the point and the viewpoint, and finds the maximum gradient among these.   If  the  cell’s
       gradient  is  higher,  it  is  marked  as visible, whereas if it is lower, it is marked as
       invisible.

       For a (square) raster of n point in  total,  the  standard  viewshed  algorithm  uses  O(n
       sqrt(n))=  O(n3/2)  time,  while  the  sweep-line  algorithm  uses  O(n  lg n) time.  This
       algorithm is efficient in terms of CPU operations and can be also made efficient in  terms
       of I/O-operations.  For all details see the REFERENCES below.

       The sweep-line.                                              The active cells.

EXAMPLES

       Using the North Carolina dataset:  Compute viewshed from a observation point (coordinates:
       638728.087167, 220609.261501) which is 5 meters above ground:
       g.region raster=elev_lid792_1m -p
       r.viewshed input=elev_lid792_1m output=elev_lid792_1m_viewshed coordinates=638728,220609 observer_elevation=5.0
       Viewshed shown on shaded terrain (observer position in the north-east quadrant with  white
       dot;  5m above ground) Using the Spearfish dataset:  calculating the viewpoint from top of
       a mountain:
       g.region raster=elevation.10m
       r.viewshed input=elevation.10m output=viewshed coordinates=598869,4916642 memory=800

REFERENCES

           ·   Computing Visibility on Terrains in External Memory. Herman Haverkort, Laura  Toma
               and Yi Zhuang. In ACM Journal on Experimental Algorithmics (JEA) 13 (2009).

           ·   Computing  Visibility on Terrains in External Memory. Herman Haverkort, Laura Toma
               and Yi Zhuang. In the Proceedings of the 9th Workshop on Algorithm Engineering and
               Experiments  /  Workshop  on  Analytic Algorithms and Combinatorics (ALENEX/ANALCO
               2007).

SEE ALSO

        r.mapcalc

AUTHORS

       Laura Toma (Bowdoin College): ltoma@bowdoin.edu

       Yi Zhuang (Carnegie-Mellon University): yzhuang@andrew.cmu.edu

       William Richard (Bowdoin College): willster3021@gmail.com

       Markus Metz

       Last changed: $Date: 2017-11-11 19:59:24 +0100 (Sat, 11 Nov 2017) $

SOURCE CODE

       Available at: r.viewshed source code (history)

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