# Autonomous Service Drones for Multimodal Detection and Monitoring of Archaeological Sites
## 2 Literature Review
This property depends on the density and composition of the material and is mostly, the reason, under the same light and temperature conditions, a stone will be warmer than the surrounding scattered ground. 3. Thermal inertia: The property that describes the ability of a material to vary its temperature more or less quickly because of changing external conditions. A high thermal inertia value corresponds to a material that is slower in cooling or heating up as external thermal conditions change. Water, for example, has a high value of thermal inertia, for this reason, a moist soil will maintain a more constant temperature than a dry soil as external conditions vary. This property is directly proportional to the volumetric heat capacity of a material and inversely proportional to its thermal conductivity. 4. Thermal emissivity: A physical quantity that measures the efficiency of a material to emit or reflect thermal radiation. This property, if noticeably different between two elements, allows a visible distinction from the thermal camera. For example, an accumulation of ceramic on the surface could be very visible in a thermal image, thanks to the different emissivity of the ceramic with respect to the surrounding soil [2; 13]. Principles explained by [PERSON] and [PERSON] in [13] refer to properties that can be found and analyzed on soils without vegetation, while in the presence of vegetation what a thermal camera can detect is not the temperature of the soil, but that of the vegetation which is determined by the transpiration of the plants. In the early 1990s, an important contribution to the use of thermography in the archaeological field came from [PERSON] et al. in [12], who wrote a complete review of all theories expressed in the previous two decades. Contrary to what [PERSON] and [PERSON] postulated, variations in the soil surface occurring over the course of a year due to vegetation growth, humidity, plowing, and other factors can hide or vary archaeological anomalies, this research demonstrated that variations in heat flow during the same day can be decisive in highlighting differences in thermal radiation due to buried archaeological elements. Some approaches proposed installing thermal sensors on satellites and civil-use aircraft. The resolution of the sensors in these attempts was poor and insufficient to detect archaeological elements. The use of airborne platforms in other approaches, such as TIMS (Thermal Infrared Multispectral Scanner), with shots taken at a much lower altitude than satellites, had a resolution of only 5 m. Yet, it allowed a single identification of an ancient road network [16]. The few examples of aerial thermography using high-resolution sensors, nevertheless, required an equipment cost that was still beyond most researchers' budgets [17]. A new development phase occurred in the early 2000s, when conventional handheld thermal cameras used by a helicopter [18] or mounted on a platform to analyze the different concentrations of artifacts scattered on the ground was introduced. This is depicted in Figure 4. Several scholars began to use the same thermal camera in different approaches. The authors in [19] mounted the camera under a kite, while authors in [20] mounted the camera under a helium balloon. The use of handheld cameras mounted on a ground-basedor an aerial platform allowed a punctual use of thermography, focused at an intra-site level. The authors in [21] were the first to use these cameras and a manned motor parachute in investigating a major research area, which also included off-site areas at a local scale. A large array of technological applications and scientific studies are supported by drones due to their ability in collecting high-resolution image data [23; 24; 25]. The recent technological advances in infrared imaging facilitated the development of high-resolution commercial thermal cameras with smaller size and weight.