# Systematic Mapping Study on Remote Sensing in Agriculture
## 2 Research Methodology
### Data Extraction Strategy
* Drones, UAVs, and manned aircraft. The use of these types of devices in agriculture has experienced a huge growth in the last decade [18]. In general, an aircraft is any vehicle which is able to fly. When they include a human pilot, they are referred as manned aircraft, while the term Unmanned Aerial Vehicle (UAV) is used when the vehicle can fly remotely (controlled by a human) or autonomously (without human control) [81]. The term drone is normally used as a synonym of UAV; however, it can also be used for other types of aquatic or land vehicles. Thus, all UAVs are drones, but not all drones are UAVs. The use of the term Unmanned Aerial System (UAS) is also frequent [97], which refers not only to the flying vehicle, but also to the ground control, communication units, support systems, etc. Compared to manned aircraft, UAVs are normally less expensive, less invasive, and safer tools, so they can be used in sensitive areas such as the polar regions [98]. The most common type of operation is the so-called visual line of sight (VLOS), where the pilot can directly see the UAV at all times; however, some systems are prepared to operate beyond visual line of sight (BVLOS) [99] allowing to cover larger extensions. * Other types of vehicles. In many cases, remote capture systems can be incorporated into the existing farm machinery [41], such as trucks, tractors, combine harvesters, etc. In this case, the images are typically used in real-time during the agricultural processes of plowing, irrigation, planting, weeding, or harvesting, more than for out-of-line analysis. We also include in this category other types of autonomous vehicles that can not be considered as UAVs, such as aerial balloons. * In-field installations. Remote image capture systems in agriculture also include field installations of fixed cameras. They can be considered _remote_ in the sense that they are used and controlled remotely, not in the capture distance. They are usually based on inexpensive cameras communicating wirelessly, which are able to perform a real-time monitoring of the crops [13]. In counterpart, they have lower resolution than the other modalities, they only capture a small portion of the plots, and normally only RGB images are used. In some cases, they can be integrated into a wider Wireless Sensor Network (WSN) installed in the farms; these include other types of sensors (thermometers, barometers, lysimeters, etc.) that are out of the scope of the present review. * **MQ7**. To date, a large number of different problems have been addressed with the RSA techniques listed above [83]. However, this fact does not limit the possibility that other new topics and areas of application will appear in the future. According to the recent reviews [17; 100], the main applications of interest can be classified as follows. * Agricultural parameters estimation. In this case, remote images are used to estimate parameters of large plots that would be difficult or expensive to be obtained using in-field methods. These parameters of interest can include crops or cropland parameters [45], for example, the height of the plants, the leaf area index (LAI), the percentage of green cover (PGC), the total biomass, the depth of the roots, or the surface roughness can be estimated. *
* Drought stress, irrigation, and water productivity. Due to the great importance of water in agriculture, this category includes all applications related to water and irrigation (although some of them could also be understood as parameter estimation) [91; 101]. Optimization of water resources is an essential aspect of global sustainability due to the great water shortage in many regions. A key parameter is water balance, which measures the water incomes and outcomes, including the crop evapotranspiration (ET). * Nutrient status. Nutrient efficiency and avoiding nutrient losses are other topics that have received much attention in the literature of RSA. The proper use of nutrients can also be aimed at reducing pollution of the environment.