Potential and limitations of LiDAR altimetry in archaeological survey. Copper Age and Bronze Age settlements in southern Iberia

The archaeological use of images and data obtained through devices carried on mobile platforms (such as airplanes and satellites) is already one century old. Today, aerial photography and remote sensing are routinely used to capture, process and analyse archaeological evidence present on the surface of the earth, which is reflected in a large body of literature—see Bewley (1999), Corsi et al. (2013), Palmer and Cox (1993), Piccarreta and Ceraudo (2000), Riley (1987) and Wilson (1982) for the former and Campana and Forte (2001), Lasaponara and Masini (2012), Limp (1989), Lyons (1981), Wiseman and El-Baz (2007) and Wheatley and Gillings (2013), for the latter. In the last two decades, there has been a steady increase in the usage of altimetric analysis based on high-resolution techniques aimed at the detection of architectural elements both above ground and underground which are difficult to detect through conventional air photography and remote sensing methods. Prominent among those techniques is airborne laser scanning (ALS), which, like terrestrial laser scanning (TLS), allows for the detection and measurement of microtopographies with a level of precision not attainable with conventional techniques of surveying and photogrammetric restitution (Challis et al., 2008; Chase et al., 2010; Doneus & Briese, 2006; Doneus & Kühteiber, 2013; Fernandez-Diaz et al., 2014; Fontana, 2022; Gallagher & Josephs, 2008; Harmon et al., 2006; Opitz, 2013; Opitz & Cowley, 2013; Risbøl, 2010; Risbøl & Gustavsen, 2018). As is well-known, this technology uses active LiDAR (light detection and ranging) sensors which emit a beam of polarized infrared light which is discretized in pulses in order to measure the distance between the sensor and the scanned object by the time difference between the pulse emission and the reception of its reflection (time of flight, TOF). This offers a value of the relative position of the object with regards the sensor, which in turn must be converted in absolute terrestrial coordinates within a geodesic system through an accurate measurement of the position, altitude, orientation and sensor speed by means of a global navigation satellite system (GNSS) with differential correction and an inertial measurement unit (IMU). When LiDAR sensors are fixed on airplanes, decimetric levels of accuracy are achieved, which may turn centimetric on helicopters or drones. The final result is a three-dimensional scatter of points which may be treated through digital 3D-modelling applications to create precise altimetric models, using both the first returns to produce a digital surface model (DSM) or the ground returns (filtered) to produce a digital terrain model (DTM) (Opitz, 2013). The application of ALS technology to extensive archaeological reconnaissance is fairly recent. Over the last decade, LiDAR has proven extremely useful, particularly in densely forested regions of northern Europe, the American continent and Southeast Asia, although its usage in Mediterranean environments is still limited. After an initial phase of testing and calibration, highly innovative and even ground-breaking results have been achieved—see, for example, Barnes (2003), Doneus and Briese (2006), Doneus (2013), Harmon et al. (2006), Challis et al. (2008), Chase et al. (2010), Risbøl (2010), Crutchley (2013), Evans (2016), Canuto et al. (2018), Historic England (2018), Guyot et al. (2021), and Prümers et al. (2022). In Spain, public, freely accessible and updated altimetric data are issued periodically since 2014, which has fostered a variety of Received: 9 September 2021 Revised: 19 May 2022 Accepted: 2 June 2022