The fate of the historic fortifications at Alcatraz island based on terrestrial laser scans and ground‐penetrating radar interpretations from the recreation yard

Abstract

ABSTRACTAlcatraz Island National Historic Landmark is best known as a former high‐security federal penitentiary that once housed many of the United States most notorious gangsters. Today, it is a popular tourist destination adjacent to the Golden Gate Bridge in scenic San Francisco Bay. Alcatraz is less known in its former military role as a 19th‐century coastal fortification protecting the interests of a rapidly westward‐expanding nation during the turbulent era of Manifest Destiny, the 1849 Gold Rush and the Civil War. The fortification, with its underground ammunition magazines and tunnels, is important from a military history perspective, marking the transition to earthen structures from the traditional brick and masonry constructions that characterized earlier 19th‐century coastal defences. In this paper, geophysical interpretations based upon an attribute analysis of ground‐penetrating radar data are combined with terrestrial laser scans, georectifications based on historical documents, maps and photographs to develop an iterative model for locating and determining the extent and integrity of subsurface historical architectural remains beneath the former recreation yard of the Alcatraz penitentiary. Using this approach, remnants of buried structures including a ‘bombproof’ earthwork traverse and its underlying vaulted brick masonry tunnel and ventilation ducts were discovered to run east‐west beneath the recreation yard. This integrative and iterative process permits accurate spatial identification of these and other 19th‐century architectural structures depicted in historical documents. This approach can be applied to subsurface investigations at other important cultural landmarks worldwide.