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r.horizon(1)		    GRASS GIS User's Manual		  r.horizon(1)

       r.horizon   -  Computes	horizon	 angle height from a digital elevation
       The module has two different modes of operation:	1. Computes the	entire
       horizon around a	single point whose coordinates are given with the 'co-
       ord' option. The	horizon	height (in radians). 2.	Computes one  or  more
       raster  maps of the horizon height in a single direction. The input for
       this is the angle (in degrees), which is	measured counterclockwise with
       east=0, north=90	etc. The output	is the horizon height in radians.

       raster, solar, sun position

       r.horizon --help
       r.horizon   [-dc]   elevation=name    [direction=float]	  [step=float]
       [start=float]	 [end=float]	 [bufferzone=float]	[e_buff=float]
       [w_buff=float]	 [n_buff=float]	  [s_buff=float]   [maxdistance=float]
       [output=basename]       [coordinates=east,north]	      [distance=float]
       [file=name]   [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

	   Write output	in degrees (default is radians)

	   Write output	in compass orientation (default	is CCW,	East=0)

	   Allow output	files to overwrite existing files

	   Print usage summary

	   Verbose module output

	   Quiet module	output

	   Force launching GUI dialog

       elevation=nameA [required]
	   Name	of input elevation raster map

	   Direction in	which you want to know the horizon height

	   Angle step size for multidirectional	horizon	[degrees]

	   Start angle for multidirectional horizon [degrees]
	   Default: 0.0

	   End angle for multidirectional horizon [degrees]
	   Default: 360.0

	   For	horizon	 rasters, read from the	DEM an extra buffer around the
	   present region

	   For horizon rasters,	read from the DEM an extra buffer eastward the
	   present region

	   For horizon rasters,	read from the DEM an extra buffer westward the
	   present region

	   For horizon rasters,	read from the DEM an  extra  buffer  northward
	   the present region

	   For	horizon	 rasters,  read	from the DEM an	extra buffer southward
	   the present region

	   The maximum distance	to consider when finding the horizon height

	   Name	for output basename raster map(s)

	   Coordinate for which	you want to calculate the horizon

	   Sampling distance step coefficient (0.5-1.5)
	   Default: 1.0

	   Name	of file	for output (use	output=- for stdout)
	   Default: -

       r.horizon computes the angular height of	terrain	horizon	in radians. It
       reads a raster of elevation data	and outputs the	horizon	outline	in one
       of two modes:

	   o   single point: as	a series of horizon heights in	the  specified
	       directions from the given point.	The results are	written	to the

	   o   raster: in this case the	output is one  or  more	 raster	 maps,
	       with each point in a raster giving the horizon height in	a spe-
	       cific direction.	One raster is created for each direction.

       The directions are given	as azimuthal angles (in	degrees), with the an-
       gle  starting with 0 towards East and moving counterclockwise (North is
       90, etc.). The calculation takes	into account the actual	projection, so
       the  angles  are	corrected for direction	distortions imposed by it. The
       directions are thus aligned to those of the geographic  projection  and
       not  the	 coordinate system given by the	rows and columns of the	raster
       map. This correction implies that  the  resulting  cardinal  directions
       represent true orientation towards the East, North, West	and South. The
       only exception of this feature is LOCATION with x,y coordinate  system,
       where this correction is	not applied.

       Using the -c flag, the azimuthal	angles will be printed in compass ori-
       entation	(North=0, clockwise).

   Input parameters:
       The elevation parameter is an input elevation raster map. If the	buffer
       options	are  used (see below), this raster should extend over the area
       that accommodate	the  presently	defined	 region	 plus  defined	buffer

       The step	parameter gives	the angle step (in degrees) between successive
       azimuthal directions for	the calculation	of the horizon.	Thus, a	 value
       of  5  for the step will	give a total of	360/5=72 directions (72	raster
       maps if used in the raster map mode).

       The start parameter gives the angle start (in degrees) for the calcula-
       tion  of	 the horizon. The default value	is 0 (East with	North being 90

       The end parameter gives the angle end (in degrees) for the  calculation
       of  the	horizon.  The  end point is omitted!  So for example if	we run
       r.horizon with step=10, start=30	and end=70 the raster  maps  generated
       by r.horizon will be only for angles: 30, 40, 50, 60. The default value
       is 360.

       The direction parameter gives the initial direction of the  first  out-
       put.  This parameter acts as an direction angle offset. For example, if
       you want	to get horizon angles for directions 45	and 225	 degrees,  the
       direction  should  be  set  to 45 and step to 180. If you only want one
       single direction, use this parameter to specify	desired	 direction  of
       horizon angle, and set the step size to 0 degrees. Otherwise all	angles
       for a given starting direction with step	of step	are calculated.

       The distance controls the sampling distance step	size  for  the	search
       for  horizon  along the line of sight. The default value	is 1.0 meaning
       that the	step size will be taken	from the raster	 resolution.   Setting
       the value below 1.0 might slightly improve results for directions apart
       from the	cardinal ones, but  increasing	the  processing	 load  of  the
       search algorithm.

       The  maxdistance	 value	gives a	maximum	distance to move away from the
       origin along the	line of	sight in  order	 to  search  for  the  horizon
       height.	 The  default maxdistance is the full map extent.  The smaller
       this value the faster the calculation but the higher the	risk that  you
       may  miss  a  terrain  feature that can contribute significantly	to the
       horizon outline.	Note that a viewshed can be  calculated	 with  r.view-

       The coordinate parameter	takes a	pair of	easting-northing values	in the
       current coordinate system and calculates	the values of  angular	height
       of the horizon around this point. To achieve the	consistency of the re-
       sults, the point	coordinate is aligned to the midpoint of  the  closest
       elevation raster	cell.

       If an analyzed point (or	raster cell) lies close	to the edge of the de-
       fined region, the horizon calculation may not be	 realistic,  since  it
       may not see some	significant terrain features which could have contrib-
       uted to the horizon, because these features  are	 outside  the  region.
       There  are to options how to set	the size of the	buffer that is used to
       increase	the area of the	horizon	analysis. The bufferzone parameter al-
       lows you	to specify the same size of buffer for all cardinal directions
       and the parameters e_buff, n_buff, s_buff,  and	w_buff	allow  you  to
       specify a buffer	size individually for each of the four directions. The
       buffer parameters influence only	size of	the read elevation map,	 while
       the  analysis  in the raster mode will be done only for the area	speci-
       fied by the current region definition.

       The output parameter defines the	basename of the	output horizon	raster
       maps.  The  raster  name	 of each horizon direction raster will be con-
       structed	as basename_ANGLE, where ANGLE is the angle  in	 degrees  with
       the  direction.	If you use r.horizon in	the single point mode this op-
       tion will be ignored.

       The file	parameter allows saving	the  resulting	horizon	 angles	 in  a
       comma separated ASCII file (single point	mode only). If you use r.hori-
       zon in the raster map mode this option will be ignored.

       At the moment the elevation and maximum distance	must  be  measured  in
       meters,	even if	you use	geographical coordinates (longitude/latitude).
       If your projection is based on distance (easting	and  northing),	 these
       too  must be in meters. The buffer parameters must be in	the same units
       as the raster coordinates (e.g.,	for latitude-longitude locations  buf-
       fers are	measured in degree unit).

       The  calculation	 method	is based on the	method used in r.sun to	calcu-
       late shadows. It	starts at a very shallow angle	and  walks  along  the
       line  of	 sight	and asks at each step whether the line of sight	"hits"
       the terrain. If so, the angle is	increased to allow the line  of	 sight
       to  pass	just above the terrain at that point.  This is continued until
       the line	of sight reaches a height that is higher than any point	in the
       region  or  until  it  reaches  the  border of the region (see also the
       bufferzone,e_buff, n_buff, s_buff, and w_buff). The the number of lines
       of sight	(azimuth directions) is	determined from	the direction and step
       parameters. The method takes into account the curvature	of  the	 Earth
       whereby	remote	features will seem to be lower than they actually are.
       It also accounts	for the	changes	of angles towards cardinal  directions
       caused by the projection	(see above).

       The examples are	intended for the North Carolina	sample dataset.

   Single point	mode
       Example	1:  determine horizon angle in 225 degree direction (output of
       horizon angles CCW from East):
       g.region	raster=elevation -p
       r.horizon elevation=elevation direction=215 step=0 bufferzone=200 \
	   coordinates=638871.6,223384.4 maxdistance=5000

       Example 2: determine horizon values starting at 90  deg	(North),  step
       size of 5 deg, saving result as CSV file:
       r.horizon elevation=elevation direction=90 step=5 bufferzone=200	\
	   coordinates=638871.6,223384.4 maxdistance=5000 file=horizon.csv

       Example	3:  test point near highway intersection, saving result	as CSV
       file for	plotting the horizon around the	highway	intersection:
       g.region	n=223540 s=220820 w=634650 e=638780 res=10 -p
       r.horizon elevation=elevation direction=0 step=5	bufferzone=200 \
	   coordinates=636483.54,222176.25 maxdistance=5000 -d file=horizon.csv
       Test point near high way	intersection (North Carolina sample dataset)

       Horizon angles for test point (CCW from East)

       We can plot horizon in polar coordinates	using Matplotlib in Python:
       import numpy as np
       import matplotlib.pyplot	as plt
       horizon = np.genfromtxt('horizon.csv', delimiter=',')
       horizon = horizon[1:, :]
       ax = plt.subplot(111, polar=True)
       bars = ax.plot(horizon[:, 0] / 180 * np.pi,
		      (90 - horizon[:, 1]) / 180 * np.pi)
       # uncomment the 2 following lines when using -c flag
       # ax.set_theta_direction(-1)
       # ax.set_theta_zero_location('N')
       Horizon plot in polar coordinates.

   Raster map mode
       Raster map mode (output maps "horangle*"	become input for r.sun):
       g.region	raster=elevation -p
       # we put	a bufferzone of	10% of maxdistance around the study area
       # compute only direction	between	90 and 270 degrees
       r.horizon elevation=elevation step=30 start=90 end=300 \
	   bufferzone=200 output=horangle maxdistance=5000

       Hofierka	J., 1997. Direct solar radiation modelling within an open  GIS
       environment.  Proceedings  of  JEC-GI'97	conference in Vienna, Austria,
       IOS Press Amsterdam, 575-584

       Hofierka	J., Huld T., Cebecauer T., Suri	M., 2007.  Open	 Source	 Solar
       Radiation  Tools	 for  Environmental and	Renewable Energy Applications,
       International Symposium on Environmental	Software Systems, Prague, 2007

       Neteler M., Mitasova H.,	2004. Open Source GIS: A GRASS	GIS  Approach,
       Springer,  New  York.  ISBN: 1-4020-8064-6, 2nd Edition 2004 (reprinted
       2005), 424 pages

       Project PVGIS, European Commission, DG Joint Research Centre 2001-2007

       Suri M.,	Hofierka J., 2004.  A New GIS-based Solar Radiation Model  and
       Its  Application	 for  Photovoltaic  Assessments.  Transactions in GIS,
       8(2), 175-190

	r.sun, r.sunmask, r.viewshed

       Thomas Huld, Joint Research Centre of the European  Commission,	Ispra,

       Tomas  Cebecauer, Joint Research	Centre of the European Commission, Is-
       pra, Italy

       Jaroslav	Hofierka, GeoModel s.r.o., Bratislava, Slovakia
       Marcel Suri, Joint Research Centre of the European  Commission,	Ispra,

       A(C) 2007, Thomas Huld, Tomas Cebecauer,	Jaroslav Hofierka, Marcel Suri	  Mar-

       Available at: r.horizon source code (history)

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       A(C) 2003-2020 GRASS Development	Team, GRASS GIS	7.8.3 Reference	Manual

GRASS 7.8.3							  r.horizon(1)


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