Watchful Realms: Integrating GIS Analysis and Political History in the Southern Maya Lowlands
Doyle, James A., Garrison, Thomas G., Houston, Stephen D., Antiquity
The use of Geographical Information Systems (GIS) has become common on archaeological projects owing to an increased interest in spatial relationships and modelling, landscape archaeology, and quantitative methods (Lock 2000; Wheatley & Gillings 2002; Conolly & Lake 2006). Aside from its cartographic functions, GIS technology offers novel approaches to the human experience of past landscapes, notably studies of visibility and pedestrian movement. Research on movement typically uses cost surface analysis (CSA) and least cost paths, which assign a cost to traversing the earth's terrain and determine likely paths for travel. In turn, visibility studies employ viewshed models to estimate the total area visible from specified points on a topographic surface (Wheatley & Gillings 2000). Both CSA and viewshed allow spatial inquiry that might otherwise be too expensive or logistically difficult to address in the field. This applies especially to the Maya lowlands, where humid subtropical forest covers large areas, land rights are contested, thus affecting access, and illegal activities, including drug trafficking and logging, inhibit ground survey. However, very few researchers have applied CSA in regional settlement analysis, despite the prevalent scholarly role that trade items and inter-site interaction play in Maya archaeology (McAnany 1989; McKillop 1996; Masson & Freidel 2002). This article applies CSA and viewshed analysis to an archaeological case study centred on the Buenavista Valley in the central southern Maya lowlands (Figure 1).
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The goal is to integrate GIS analyses with settlement survey and epigraphic data to evaluate possible spatial scenarios for waxing and waning control of exchange routes between two Classic Maya polities (c. AD 250-900). The analysis utilises CSA and viewshed models for known periods of conflict and political aggression, especially between the Calakmul and Tikal polities that dominated dynastic interaction during the Late Classic period (c. AD 550-800). Such analysis demonstrates the value of CSA and viewshed methods in addressing archaeologically the impact of communication routes and visibility on ancient populations, and probes more deeply into dynamic, conflictive political landscapes of the ancient world.
Modelling movement and visibility
The principle governing CSA is that two locations on the earth's surface may be the same linear distance from one another but unequal in the amount of effort or time needed to reach the other when travelling. A series of GIS methods creates raster images that assign a cost to each pixel, costs that accumulate in the digital elevation model (DEM) from a fixed point of departure. The costs are generally determined by slope, based on the premise that human physiology favours slopes at differential rates. After computing the cost of travelling over each pixel, GIS software generates a line vector representing the 'least cost path' traversing neighbouring cells with the lowest value (Madry & Rakos 1996:113; Bell & Lock 2000; Harris 2000; Bell et al. 2002; Howey 2007).
CSA cost can be considered isotropic, or the same in all directions, partially anisotropic, or operating from a particular direction, or fully anisotropic, wherein change in direction of travel increases or decreases cost (Conolly & Lake 2006: 215; Bevan 2008: 4). Anisotropic cost surfaces based on slope often factor in aspect (direction), because cost increases when moving uphill, but decreases on the downward slope (Krist & Brown 1994; Bell & Lock 2000: fig. 4; Llobera 2000; van Leusen 2002: 5-6). A non-linear formula for calculating cost involves dividing the tangent of the angle of the pixel slope by the tangent of 1[degrees], creating a cost surface that reflects the "comparative difficulty in traversing steep terrain" (Bell & Lock 2000: 88-89). …