A transmission tower (electricity pylon in the United Kingdom and parts of Europe, and a hydro tower in certain provinces of Canadawhere power generation is mainly hydro-electric) is a tall structure, usually a steel lattice tower, used to support an overhead power line. They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes. Typical height ranges from 15 to 55 metres (49 to 180 ft), though the tallest are the 370 m (1,214 ft) towers of a 2700-metre-long span of Zhoushan Island Overhead Powerline Tie. In addition to steel, other materials may be used, including concrete and wood.There are four major categories of transmission towers: suspension, terminal, tension, and transposition. Some transmission towers combine these basic functions. Transmission towers and their overhead power lines are often considered to be a form of visual pollution. Methods to reduce the visual effect include undergrounding.