Probabilistic Modelling using Monte Carlo Simulation for Incorporating Uncertainty in Least Cost Path Results: a Postdictive Roman Road Case Study
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 fully 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. By incorporating vertical error into the LCP modelling process the accuracy of the LCP results can be understood probabilistically, with the likelihood of obtaining an LCP result quantified. Furthermore, the effect of incorporating vertical error on the LCP results can be expressed through the use of probabilistic LCPs, allowing for a graphical representation of the uncertainty in the LCP calculation, as well as identifying the most probable location of the ‘true’ least cost path. The method of understanding LCP results probabilistically is applied to a Roman road case study, finding that the accuracy of the LCP from south-to-north without incorporating vertical error is not representative of the LCP population with vertical error accounted for. In contrast, the accuracy of the LCP without incorporating vertical error from north-to-south is representative of the LCP population. The use of probabilistic LCPs suggests that the location of the Roman road in the southern section of the study area was selected to minimise the time taken to move up and down slope, irrespective of the direction of movement. However, the identification of two corridors of similar likelihood of containing the ‘true’ location of the LCP in the northern section when modelling movement south-to-north suggests that the input data and parameters used in the LCP analysis are unable to discern which corridor contains the most probable ‘true’ location of the LCP. Therefore, this research suggests that different input data and parameters are used and tested.