IPW User Command
Category - Topographic Calculations
horizon - find angles to local horizon along rows of elevation file
horizon -a azimuth [ -d delta ] [ -z zen ] [ -u cos ] [image]
Horizon is part of a suite of radiation, solar, and topographic geometry tools that are part of the IPW system
Horizon computes the local horizon angles toward the direction azimuth, where azimuth=0 is toward the south and positive angles are counter-clockwise.
horizon reads elevations from image (default: standard input) and writes to the standard output an image whose pixels encode the local horizon angles in the direction azimuth degrees (ranging from -180 to 180) from south (positive east). The value of each output pixel is the cosine of the angle from the zenith to the pixel's horizon in the forward (increasing sample coordinates) direction. (Note than this value is also the sine of the angle from true horizontal to the pixel's horizon.)
The following options change the output from linearly quantized
cosines to a 1-bit mask in which 1's indicate horizon angles
greater than a specified threshold. They are typically used to
specify a solar zenith angle, the output being a mask of pixels
where the sun is visible.
To compute northwest horizons:
horizon -a -135
To produce a mask of all northwest horizon angles greater than 45 degrees:
horizon -a -135 -z 45
(i.e., any pixels that would be shadowed by adjacent terrain at this solar zenith and azimuth would be masked as 0.)
$WORKDIR/horizonNNNNN temporary command file, removed when horizon exits
horizon is a shell script than skews and/or transposes the input image to orient its scan lines in the direction azimuth, then calls hor1d to perform the actual horizon calculations.
None that we know of
Marks 1979, Dozier 1980, Dozier Bruno 1981, Dozier 1990, Dubayah 1990, Frew 1990, Marks 1991