The movement of past peoples in the landscape has been studied extensively through the use of least cost path (LCP) analysis. Although methodological issues of applying LCP analysis in archaeology have frequently been discussed, the effect of DEM error on LCP results has not been fully assessed. Due to this, the reliability of the LCP result is undermined, jeopardising how well the method can confidently be used to model past movement. To strengthen the reliability of LCP results, this research proposes the use of Monte Carlo simulation as a method for incorporating and propagating the effects of error on LCP results. Focusing on vertical error, random error fields are calculated and incorporated into the documented and reproducible LCP modelling process using the R package leastcostpath. By graphically communicating the impact of vertical error using probabilistic LCPs, uncertainty in the results can be taken into account when interpreting LCPs. The method is applied to a Roman road case study, finding that the incorporation of vertical error results in the identification of multiple ‘least cost’ routes within the landscape. Furthermore, the deviation between the roman road and the probabilistic LCP suggests that the location of the roman road was influenced by additional factors other than minimising energy expenditure. This research finds that the probabilistic LCP derived using Monte Carlo simulation is a viable method for the graphical communication of the uncertainty caused by error within the input data used within the LCP modelling process. Therefore, it is recommended that probabilistic LCPs become the default approach when modelling movement using input data that contains errors.

The movement of past peoples in the landscape has been studied extensively through the use of Least Cost Path (LCP) analysis. Although methodological issues of applying LCP analysis in Archaeology have frequently been discussed, the effect of vertical error in the DEM on LCP results has not been assessed. This research proposes the use of Monte Carlo simulation as a method for incorporating and propagating the effects of vertical error on LCP results. Random error fields representing the vertical error of the DEM are calculated and incorporated into the documented and reproducible LCP modelling process using the R package leastcostpath. The development of probabilistic Least Cost Paths allows for the identification of areas that are probabilistically more likely, and likewise less likely, to have an LCP crossing a cell. Furthermore, by incorporating uncertainty into the LCP modelling process, greater confidence can be given to LCP results. These methods are applied to a Roman road case study that aims to illustrate how interpretations drawn from a single realisation of an LCP should be viewed with caution. Instead, results should be based on multiple realisations of the LCP after incorporating vertical error and interpretations should be expressed probabilistically through the use of probabilistic LCPs.

The Gask Ridge system is a series of forts, fortlets, and timber watchtowers situated along a Roman road in northern Scotland. The high intervisibility of the watchtowers in the Gask Ridge system has resulted in the proposal of two main functions: one that the watchtowers were a signalling system and two that the watchtowers provided visibility of the Gask Ridge road for surveillance and monitoring.Despite this, only the former function has been assessed. This paper explores the function of the watchtowers along the Gask Ridge road using computational methods, including Monte Carlo hypothesis testing. The analytical approach, which is documented and reproducible with accompanying code, rejects that the watchtowers were randomly located along the Gask Ridge road, instead favouring the alternative hypothesis that the watchtowers were located to maximise the visibility of the road. Furthermore, it is possible to claim that the need to monitor the road shows a causal relationship with the location of the watchtowers, rather than associative. The findings support the interpretation that main function of the watchtowers was for the surveillance and monitoring of the Gask Ridge road, providing an early warning system of an attack from the Highlands (Woolliscroft, 1993).

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The large-scale provision of defenses around small towns in Roman Britain during the 2nd c. CE is without parallel in the Roman Empire. Although the relationship between defended small towns and the Roman road network has been noted previously, provincial-level patterns remain to be explored. Using network analysis and spatial inference methods, this paper shows that defended small towns in the 2nd c. are on average better integrated within the road network – and located on road segments important for controlling the flow of information – than small towns at random. This research suggests that the fortification of small towns in the 2nd c. was structured by the connectivity of the Roman road network and associated with the functioning of the cursus publicus.