An observation tower is a tower built wholly or partly to provide an elevated vantage point from which people can look out over the surrounding landscape or cityscape, usually through an unobstructed 360-degree field of view. Unlike a watchtower, which is occupied for surveillance or fire detection over an extended period, a dedicated observation tower exists principally so that visitors may enjoy or study a panoramic view. Observation towers are typically at least tall, so that they clear the surrounding tree canopy or built environment, and have historically been constructed in stone, brick, timber and iron; reinforced concrete, structural steel and glued laminated timber predominate in modern examples.
Because a tall structure is expensive to build and maintain for viewing alone, observation functions are very often combined with another use. Many of the world's best-known observation towers are also broadcasting towers, and observation decks are routinely incorporated into water towers, church steeples, lighthouses, high-rise buildings and even the pylons of bridges. Admission fees and tower restaurants help to offset construction and running costs, and in many cities the tower has become a landmark and a symbol of civic identity.
Structures closely related to the observation tower include the crow's nest, the belvedere, the viewing platform, the observation deck, the gyro tower and the observation wheel.
Terminology
The English term observation tower covers a broad family of structures, and usage overlaps with several near-synonyms. In German-speaking Europe, where the type is especially common, a purpose-built viewing tower is an Aussichtsturm, distinguished from the Wachturm (watchtower) and the Fernsehturm (television tower). A belvedere—from the Italian for "beautiful view"—is generally an architectural pavilion or gallery placed to command a prospect rather than a free-standing shaft, while a folly tower or prospect tower is an ornamental structure on a landscaped estate built at least partly for effect.
A tower is usually described as an observation tower only where public or semi-public access to a viewing level is intended. Where the elevated position exists for an operational purpose—air traffic control towers, fire lookouts, prison and border guard towers, lifeguard towers—the structure is more precisely a watchtower or lookout, even though the two categories frequently exchange members: decommissioned fire lookouts are often reopened as public viewpoints, and some public towers are pressed into service as lookouts during periods of high fire risk.
Construction and usage
Observation towers are conspicuous features of the landscape, since they must rise above trees, terrain and neighbouring buildings to deliver a clear view. Siting is therefore as important as height: a modest timber tower on a summit or ridge may command a far wider prospect than a much taller structure in a valley or dense urban district.
Structural forms
Four structural systems account for the great majority of observation towers:
Masonry towers — the traditional form of the 18th- and 19th-century prospect tower, built of quarried stone or brick with a solid or cavity wall and an internal spiral stair. Masonry towers are durable and require little maintenance but are heavy, slow to build and practically limited in height. Lattice and framed towers — open frameworks of wrought iron, steel or, increasingly, timber. Lattice construction offers a high strength-to-weight ratio, transparency to wind and comparatively low material cost. The Eiffel Tower (1889) is the archetype; a distinct sub-family is the hyperboloid structure, pioneered by Vladimir Shukhov, in which straight members arranged as a doubly ruled surface produce a curved, highly efficient shell. Reinforced concrete shafts — the standard solution for tall post-war broadcasting and observation towers. A slipform-cast hollow shaft carries lifts, stairs and services, with a cantilevered "turret" or "pod" housing decks, restaurants and antenna galleries. Concrete supplies the mass and stiffness needed to limit sway and to support heavy antenna loads. Timber towers — historically common for cheap rural lookouts and now enjoying a revival through glued laminated timber and cross-laminated timber. The tower on the Pyramidenkogel in Carinthia, Austria, completed in 2013, reaches to the tip of its antenna; its operators describe it as the tallest timber viewing tower in the world, and its structure consists of sixteen elliptically arranged larch glulam columns braced by steel diagonals.
Engineering considerations
Slender towers are governed less by gravity loads than by lateral loads and by human comfort. Wind is the dominant design case: designers must resist both the mean drag force and dynamic effects such as vortex shedding, which can excite crosswind oscillations in a lightly damped shaft. Countermeasures include aerodynamic shaping, helical strakes, open lattice work and tuned mass dampers; the Tokyo Skytree additionally employs a central concrete core linked to the outer steel frame in a vibration-control system inspired by the shimbashira of traditional Japanese pagodas. In seismic regions, ductile detailing and base isolation are used to accommodate ground motion. Perceptible sway at the top of a tall tower is normal and is limited by comfort criteria rather than strength: occupants can typically detect accelerations well below any level that threatens the structure.
Other recurring design problems include lightning protection, ice accretion and the risk of ice falling from high decks and antennas, condensation and cleaning of glazing, evacuation from great heights with limited stair capacity, and the reliability of lifts, which are usually the only practicable means of access above about .
Access and amenities
Access to a viewing level may be by stair, lift, or both. Purpose-built rural towers are almost always climbed on foot; broadcasting towers and high-rise decks rely on high-speed lifts, often with a stair retained solely for emergency escape. Some towers have turned the climb itself into an attraction—the CN Tower's internal stairway of 2,579 steps is used for charity climbs—while others offer novel descents, such as the slide at the Pyramidenkogel.
Viewing levels may be open-air platforms, glazed galleries, or a combination. Open platforms are preferred by photographers because there are no reflections or dirty glazing to shoot through, whereas enclosed galleries are more comfortable in wind, cold or rain and permit year-round operation. Later additions to the repertoire include glass floors, cantilevered skywalks, external walkways with harness systems, and abseil or bungee attractions. Restaurants are common at larger towers and are sometimes built as revolving restaurants, a form popularised from the late 1950s onward.
Access arrangements vary widely. Small rural towers are often permanently open and free of charge; municipal and association-owned towers may open only at weekends or in season; and commercially operated decks on broadcasting towers and skyscrapers charge an admission fee and keep fixed hours. Ticket revenue can be substantial and, at broadcasting towers, is frequently used to subsidise the upkeep of transmission infrastructure.
Types
Dedicated observation towers
Particularly in the decades before World War I, rambling and alpine associations, beautification societies and municipalities built observation towers on large numbers of summits and hilltops across central Europe. Most were of stone, though timber and iron were also used, and access to the platform—typically between above ground—was almost always by stair. Many such towers remain in the ownership of the clubs that built them and are maintained by volunteers.
Although tourism was the primary purpose, secondary uses were common. Towers were pressed into service as fire lookouts in dry seasons and as military observation posts, sometimes with anti-aircraft positions alongside, in wartime. Today a number carry antennas for emergency-service radio, mobile telephony or low-power FM and television transmitters. A flagpole at the summit is a characteristic feature of older examples.
Some of these towers are permanently accessible, either free or on payment of an admission fee; others open only at set times. The platform is usually open to the air, and a café or inn at the base is frequent. A few have far more extensive accommodation: the tower on the Rossberg near Reutlingen in Germany incorporates a hotel within its structure.
Construction of purpose-built viewing towers did not stop in 1914, and new examples continue to appear, often as attractions at horticultural shows and regional garden festivals, as landmarks for nature reserves and country parks, and as elements of landscape art programmes. Brick has largely given way to concrete, steel and engineered timber, and contemporary designs are frequently the outcome of architectural competitions; several have won major design awards, among them the 23.5-metre timber hyperboloid tower at Hemer in Germany, built for a regional garden show.
Permanent observation towers are occasionally found in amusement parks, though in parks with a single all-inclusive admission charge, ride-based panorama towers and observation wheels are generally preferred because they move visitors through in a fixed cycle.
Watchtowers
Watchtowers are observation towers from which a person supervises an area. Strictly speaking control towers belong to this category, although at airports much of the surveillance is now carried out by radar and other non-optical means. Watchtowers usually have an enclosed cabin to protect the observer from the weather, and because they are seldom more than about tall they rarely have a lift. Active watchtowers are not generally open to the public, since they monitor sensitive areas; however, some fire watchtowers include a public platform, or are opened to visitors outside the fire season. Decommissioned watchtowers are readily converted into observation towers, and many former border, prison and military towers have been preserved as viewpoints or as memorials.
Radio and television towers
Some radio towers are designed so that, in addition to carrying transmitting equipment, they can be used as observation towers. This requires a structure stable enough to permit continuous safe public access without interrupting transmission. It is readily achieved for UHF and VHF services, whose antennas sit at the top of an earthed structure, but not for most long-wave and medium-wave installations, in which the mast itself radiates and is electrically live.
The difficulty is illustrated by the Berlin Radio Tower, which originally carried a medium-wave T-antenna together with an 80-metre mast and stood on insulators. Early test transmissions showed that voltages at the tower would be hazardous to visitors, and the tower was therefore earthed through its lift shaft—at the cost of directing the main beam away from the intended service area, the city of Berlin. Since almost all pre-war broadcasting used the long-, medium- and short-wave bands, towers acting purely as antenna supports only became widespread after World War II with the introduction of VHF and UHF services; it was these that were routinely given observation decks, most often using closed reinforced-concrete construction.