# Exploring Ephemeral Features with Ground-Penetrating Radar: An Approach to Roman Military Camps
## 4 Discussion  
The lucky coincidence of several Roman camps being excavated simultaneously during 2021 in different areas of north-western Iberia allowed us to carry out an experimental, small-scale GPR survey to assess their potential outcomes. The overall results of GPR survey on ephemeral Roman army traces were deemed positive since the dug-out defensive structures were easily sensed in all of them. The geophysical survey in Alto da Raia allowed us to map a previously unknown built feature. This canonical _clavicula_ gate will be subject to further investigation in future archaeological works. However, this approach works best in combination with other techniques. The methodology for studying all the sites presented above comprised remote sensing techniques (LiDAR, aerial/satellite imagery, and UAV mapping), geophysical surveys (magnetometry and GPR), and archaeological excavation and sampling (Figure 15). The research carried out in Alto da Raia, Sueros de Cepeda, and the siege of Cerro de Castarreno benefited from a coordinated strategy in which geophysical prospection and excavation were carried out under the same premises. This initiative laid its foundations in the experience acquired after a decade of archaeological projects exploring different methodologies for studying archaeological features in north-western Iberia [4; 49]. From a technical point of view, this experimental approach allows us to conclude that it is possible to gather valuable data for identifying ephemeral features related to short-term Roman military occupations in the different environments within the study  
Figure 15: (**a**) Excavation at Alto da Raia (photography courtesy of Era-Arqueologia); (**b**) Excavation trench at Sueros de Cepeda (photography: DGA); (**c**) Excavation trench at Villa Maria (photography JMCG); (**d**) Excavation trench at Los Andinales (photography JMCG). area. In applying GPR surveys, traditional interpretation based on primary products such as slice layers and radargrams could be complemented by the volumetric visualisation of low-amplitude signals. More refined processing methods such as Hilbert transformation should be explored to isolate subtle changes in soil composition. Once again, recent methodological developments reveal how traditional pessimism regarding identifying the ancient traces of the Roman military deployment in north-western Iberia has been definitively overcome. This methodological leap forward is causing a profound transformation in the archaeological debates devoted to the Roman expansion in the region, overcoming traditional pessimism [50; 51] about the possibility of identifying material traces related to Roman military activities of temporary nature. More importantly, the results of this paper support the potential of new non-invasive methodologies to produce new interpretations of the Roman-indigenous conflict beyond the textual evidence provided by the ancient Greek and Latin authors. Optimism does not prevent us from being aware of the diverse techniques' limitations. The resolution of the GPR data did not allow us to obtain exact information about the features' geometry, a relevant feature to assess Roman army strategy and the techniques employed in the construction of camps and siege. That information was later achieved through archaeological excavation. This lack of precise geometric information from the Roman trenches might be related to the GPR data acquisition process: the survey sometimes aligned with the trenches due to present-day ground conditions, i.e., crops and plough lines. Nevertheless, even aligned GPR passes could map the horizontal shape of these infills in a very clear way. The case studies' diverse geologic and lithologic contexts do not seem to have influenced the data quality. The clayish and sandy soils produce good results with a clear contrast between the soil matrix and trenches, as seen in Villa Maria and Los Andinales. In Alto da Raia, the slate and granitic geology also offer a favourable scenario for the GPR survey.