Assessment of the Permeability of Historical Defensive Systems: the Case of the Lines of Torres Vedras

As the largest part of military usage of the territory depends on a thorough analysis of its morphology, spatial analysis with Geographic Information Systems (GIS) software reveals a great potential as a way for studying battlefields and/or other military buildings and scenarios from the past, while supporting interpretations of territorial occupation and related organization of military forces through event simulation. This paper presents a GIS-based method to model and assess the permeability of historical defensive systems from the period that preceded the mechanization of warfare of the late nineteenth century, i.e., based on artillery installed in fortifications. The method was applied to analyze the permeability of the Lines of Torres Vedras (LTV) defensive system, a multi-line fortification complex which was operational during the Peninsular War in the beginning of the nineteenth century and located north of Lisbon, Portugal. The structures of the LTV and local terrain were modelled in GIS using a set of proposed parameters that support territorial analysis, combined with the capability of contemporary weaponry. A methodical quantification and combination of these parameters enabled a better understanding of the systemic design of the defensive lines in terms of its permeability, contributing to a discussion of the LTV particular military setting based on the interpretation of these indicators. Global and sectorial analyses of the permeability of the LTV defensive system were conducted.