# Exploring Ephemeral Features with Ground-Penetrating Radar: An Approach to Roman Military Camps
## 2 Materials and Methods  
One of the main goals of this paper is to introduce a workflow based on non-invasive approaches with ground-truthing. The exponential growing interest in the Roman conquest of the Iberian northwest has precipitated the development and application of a common research programme and the combined analysis of study cases at different levels [7; 14; 15]. The intended methodology aligns with existing archaeological approaches to studying the Roman military presence elsewhere in Europe [16; 17; 18; 19; 20; 21]. This workflow comprises four consecutive stages: (1) remote sensing; (2) geophysical survey; (3) ground-truthing via excavation; (4) sampling and archaeometric analyses. On this occasion, we will focus on the first three stages.  
The first stage of studying Roman military landscapes relies on remote sensing technologies (namely airborne LiDAR, aerial photography, and satellite imagery). LiDAR has revolutionised archaeological research in the Iberian Peninsula, where upland and forested regions were previously ill-explored by scholars. New archaeological evidence was brought to light thanks to the critical analysis of LiDAR-derived datasets from two different coverages from the Spanish National Geographic Institute (IGN) through the PNOA project (National Aerial Orthophotography Plan). This phenomenon challenged the traditional narratives on the expansion of the Roman state and led to a completely renewed archaeological scenario [4; 6; 7; 22; 23; 24]. As is often the case, new evidence raises new research questions--notably the chronological sequence of those features that appear as palimpsests on the landscape still needs to be clarified.  
The potential of LiDAR datasets for detecting Roman military sites is severely limited in flattened areas or large agricultural fields where the archaeological sites rarely retain topographic expression [25]. That is precisely the case for many territories located north of the Douro River, which comprise a vast sector of our study area. The research strategy followed a two-step process when dealing with these plain areas [7]. Firstly, crop marks were detected using national and international aerial and satellite coverages such as those from the Spanish IGN-PNOA or private, commercial missions. Secondly, the acquisition of high-resolution UAV images led to generating photogrammetric RGB and infrared orthophotos and digital surface models (DSM). While the imagery provided by public or private entities is produced at any time of the year, the UAV flights are carefully planned according to the best moments of the crop cycle to maximise information. Nevertheless, conditions can easily vary due to anthropogenic and natural causes, the reason why the surveys are carried out yearly. The increasing spatial, spectral, and temporal resolution of the imagery coverage leads to more accurate documentation of the archaeological features and the potential discovery of new sites.  
This paper presents the application of GPR to counter the limitations and variability of the different remote sensing approaches to studying Roman military camps in northwestern Iberia (Figures 1 and 2; see Table 1). We aim to increase our understanding of previously mapped structures and detect new features that could help better characterise these sites, i.e., defensive elements guarding the gates. Although other geophysical techniques (fluxgate magnetometer) were used to explore these sites during our field activities,we will focus on the contributions of GPR since its use to document ephemeral structures in Roman military contexts is not as usual [13; 26; 27].