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e useful for aviation purposes.
Global Mapper User's Manual
Optionally, you can also specify that the receiver elevation should be calculated based on an elevation
angle relative to the horizon from the transmitter. This is useful if you have something like a radar
dish that points up at some angle and you want to see where the signal can be seen.
Finally, you can also specify a transmission angle range for a beam transmitted from the transmitter.
Then the view shed will depict where that beam would hit the terrain surface (or some user-specified
distance above the surface).
The Select Elevation Layer(s) to Base Transmitter/Receiver Heights On button displays a dialog
allowing you to select which of the loaded elevation layers you want to base ground-relative
transmitter and receiver heights on. The default is to use all loaded layers, but if you have a situation
where you have a ground level data set loaded and perhaps another set with heights of buildings, etc.,
you could use this option to cause the transmitter and receiver heights to be based on the ground
elevation, whereas the actual visibility of each point will use the topmost of any loaded layer.
The View Radius section allows the user to specify how far in each direction from the transmitter to
check for visibility. Typically you'd want to set this to the effective range of your transmitter. If you
want to ignore areas close to the transmitter, you can also specify a minimum view radius value. Use
the default of 0 to include everything from the transmitter out ot the selected view radius.
The View Angle section allows the user to limit the view shed to a particular subsection of the
complete radial area. The Start Angle specifies the cartographic angle at which the radial subregion
begins. This angle is a cartographic angle, meaning that 0 degrees is north and angles increase
clockwise. The Swept Angle specifies the number of degrees clockwise to include in the view shed.
For example, if the transmitter being analyzed sweeps an arc from due south to due west, a start angle
of 180 with a swept angle of 90 would be used. To perform a view shed analysis over the entire area,
keep the defaults of starting at 0 degrees and sweeping through 360 degrees.
The Earth Curvature section allows the user to specify whether they want to take the curvature of the
earth into account while performing the view shed analysis. In addition, when earth curvature is being
used, they can specify an atmospheric correction value to be used. The atmospheric correction value
is useful when determining the view shed for transmitting waves whose path is affected by the
atmosphere. For example, when modeling microwave transmissions a value of 1.333 is typically used
to emulate how microwaves are refracted by the atmosphere.
The Sample Spacing section allows the user to specify the spacing of elevation samples when
calculating the view shed. The sample spacing controls the interval at which elevation samples are
examined to determine visibility. Smaller values result in more accurate, but more slowly generated,
view sheds.
The Fresnel Zone Specification section allows you to have the view shed analysis also check that a
certain portion (the Percent Clear value) of the first Fresnel zone for a transmission of a particular
frequency is clear. The typical standard is that good visibility requires that at least 60% (the default)
of the first Fresnel zone for the specified frequency be clear of obstructions. If you specify a
maximum Fresnel zone percentage clear other than 100%, only those locations where the minimum
percentage of the 1st Fresnel zone that is clear is between your specified percentages will be marked
as visible.
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