Provided by: grass-doc_6.4.3-3_all 
NAME
r.horizon - Horizon angle computation from a digital elevation model.
Computes horizon angle height from a digital elevation model. The module has two different
modes of operation: 1. Computes the entire horizon around a single point whose
coordinates are given with the 'coord' 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.
KEYWORDS
raster, sun position
SYNOPSIS
r.horizon
r.horizon help
r.horizon [-d] elevin=string [direction=float] [horizonstep=float] [bufferzone=float]
[e_buff=float] [w_buff=float] [n_buff=float] [s_buff=float] [maxdistance=float]
[horizon=string] [coord=east,north] [dist=float] [--verbose] [--quiet]
Flags:
-d
Write output in degrees (default is radians)
--verbose
Verbose module output
--quiet
Quiet module output
Parameters:
elevin=string
Name of the input elevation raster map [meters]
direction=float
Direction in which you want to know the horizon height
horizonstep=float
Angle step size for multidirectional horizon [degrees]
bufferzone=float
For horizon rasters, read from the DEM an extra buffer around the present region
e_buff=float
For horizon rasters, read from the DEM an extra buffer eastward the present region
w_buff=float
For horizon rasters, read from the DEM an extra buffer westward the present region
n_buff=float
For horizon rasters, read from the DEM an extra buffer northward the present region
s_buff=float
For horizon rasters, read from the DEM an extra buffer southward the present region
maxdistance=float
The maximum distance to consider when finding the horizon height
horizon=string
Prefix of the horizon raster output maps
coord=east,north
Coordinate for which you want to calculate the horizon
dist=float
Sampling distance step coefficient (0.5-1.5)
Default: 1.0
DESCRIPTION
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:
single point: as a series of horizon heights in the specified directions
from the given point. The results are written to the stdout.
raster: in this case the output is one or more rasters, with each point in
a raster giving the horizon height in a specific direction. One raster is
created for each direction.
The directions are given as azimuthal angles (in degrees), with the angle 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.
Flags:
-d
Output horizon height in degrees (the default is radians)
Input parameters:
The elevin 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 zones.
The horizonstep parameter gives the angle step (in degrees) between successive azimuthal
directions for the calculation of the horizon. Thus, a value of 5 for the horizonstep will
give a total of 360/5=72 directions (72 rasters if used in the raster mode).
The direction parameter gives the initial direction of the first output. 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 horizonstep to 180.
If you only want one single direction, use this parameter to specify desired direction of
horizon angle, and set the horizonstep size to 0 degrees.
The dist 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 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.
The coord 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 results, 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 defined region, the
horizon calculation may not be realistic, since it may not see some significant terrain
features which could have contributed 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 allows 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 specified by the current region
definition.
The horizon parameter gives the prefix of the output horizon raster maps. The raster name
of each horizon direction raster will be constructed as horizon_NNN , where NNN counts
upwards from 0 to total number of directions. If you use r.horizon in the single point
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.
METHOD
The calculation method is based on the method used in r.sun to calculate 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 horizonstep 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).
EXAMPLE
Single point mode:
r.horizon elevin=DEM horizonstep=30 direction=15 bufferzone=200 \
coord=47.302,7.365 dist=0.7 > horizon.out
Raster map mode (for r.sun):
# we put a bufferzone of 10% of maxdistance around the study area
r.horizon elevin=DEM horizonstep=30 bufferzone=200 horizon=horangle \
dist=0.7 maxdistance=2000
SEE ALSO
r.sun, r.sunmask, r.los
REFERENCES
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
AUTHORS
Thomas Huld, Joint Research Centre of the European Commission, Ispra, Italy
Tomas Cebecauer, Joint Research Centre of the European Commission, Ispra, Italy
Jaroslav Hofierka, GeoModel s.r.o., Bratislava, Slovakia
Marcel Suri, Joint Research Centre of the European Commission, Ispra, Italy © 2007, Thomas
Huld, Tomas Cebecauer, Jaroslav Hofierka, Marcel Suri Thomas.Huld@jrc.it
Tomas.Cebecauer@jrc.it hofierka@geomodel.sk Marcel.Suri@jrc.it
Last changed: $Date: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $
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