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Line of sight is the direct free-space path that exists between two points.
Using binoculars on a clear day, it is easy to determine if visual line of sight exists between two points that are miles apart. To have a clear line of sight there must be no obstructions between the two locations. Often this means that the observation points must be high enough to allow the viewer to see over any ground-based obstructions. If any of these obstructions rise high enough to block the view from end to end, there is no visual line of sight. Obstructions that can interfere with visual line of sight can also interfere with radio line of sight.