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A reasonable case exists for seeing John Claudius Loudon as Landscape Architecture's Giant. He lived from 1783-1843. More research is necessary but it is evident that:
(1) Loudon  was nearer to polymath status than anyone else who has devoted their life to the profession
(2) Loudon had a decisive influence on the adoption of the term 'landscape architect' by the profession
(3) Loudon laid the ground for a science-and-art based profession specialising in public projects (rather than private gardens)
(4) Loudon's 1822 and 1829 proposals for London (which included a circular Promenade and a set of concentric Breathing Zones, which we would call Green Belts or Green Infrastructure - GI) had a far-reaching influence which may well have included subsequent proposals by William Light (for Adelaide), by Joseph Paxton (for London),  by Frederick Law Olmsted (for Boston), Ebenezer Howard's green belt ideas and Patrick Abercrombie's County of London Plan and Greater London Plan.
(5) Loudon is one of great 'fathers of landscape architecture'.