Balloons.

This is coolbert:

Man in a basket. 

Observation balloons used extensively by all combatants during the Great War [WW1], a tethered balloon raised aloft hanging from which is a basket with a manned observer. This was a common sight near the front lines during that conflict.

"Observation balloons are balloons that are employed as aerial platforms for intelligence gathering and artillery spotting. Their use . . . reaching their zenith during World War I"

Make no mistake about it, this was very dangerous duty, either to be an occupant of the basket, the observer slung from underneath the inflated and carried aloft balloon, or that enemy pilot approaching and desiring a shoot-down that target so tempting but nonetheless heavily defended.

"Because of their importance as observation platforms, balloons were heavily defended by anti-aircraft guns and patrolling fighter aircraft. Attacking a balloon was a risky venture, but some pilots relished the challenge."

Balloons inflated with highly explosive and flammable hydrogen. Tracer rounds from an attacking aircraft able to set alight the balloon with disastrous consequences.

Man in the basket, the observer being able to parachute to ground if and when an enemy attack successful, the balloon burst and dropping from the sky. Attacking enemy combat aircraft subjected to massed and dedicated AAA [anti-aircraft-artillery] fire, withering and intense. DANGEROUS DUTY FOR ALL PARTIES AND THE HAZARD NEVER MINIMAL!!

From an altitude of 2,000 feet  [600 meters] the line-of-sight [LOS] of the observer on a good day at least sixty miles [100 kilometers].

From the era of the Great War a view as might have been seen 

from a balloon. Those rectangles are farm fields.

Tethered balloons under some circumstances still an option for the military and security apparatus of a quasi-military nature. NOW sans basket but with high-definition telephoto television lenses, day/night infra-red capability, and MUCH MORE, helium and not hydrogen the gas of choice now!!

coolbert.