Issue link: http://winesandvines.uberflip.com/i/987926
70 WINES&VINES June 2018 GRAPEGROWING WINE EAST images of large areas, but the resolution is generally not detailed enough for precision viticulture applications. 18 The spatial resolution of aerial images acquired by satellites or manned aircraft is optimally 20-50 centimeters per pixel. 33 UAVs have a higher flexibility of use and lower operational costs, and due to a UAV's ability to fly lower to the ground than other airborne platforms, the resolution of the aerial images can be much higher than with conventional aircraft or satellites. The resolu- tion of UAV-acquired images can be as high as 1 cm. per pixel. Matese et al. 18 compared three remote sens- ing platforms (UAVs, aircraft, satellites) in terms of their technical, scientific and eco- nomic performance. Aerial surveys were per- formed by all three platforms to access their ability to assess spatial variability of vegetation in two vineyards in the Veneto region of Italy. They characterized advantages and disadvan- tages into two main categories: mission and processing. The mission category covers planning and execution of the aerial surveys. These include: ability to deal with different weather condi- tions and the scheduled practices of the vine- yard; ability for the platform to reach the site being surveyed; the need for multiple flights to acquire the whole area of interest, and reli- ability of the platform. Compared to satellites or aircraft, UAVs operate closer to the ground (90-120 m; ≈400 ft.), have more flexibility in scheduling, and are not affected by cloud cover. However, UAVs have a much shorter range and less endurance than aircraft and satellites. The processing category includes computa- tion factors needed to transform the raw im- ages into a final product. These factors include payload, resolution, precision, mosaicking (whereby a single image is created from mul- tiple images) and geocoding efforts and pro- cessing time. The advantages of UAVs over aircraft or satellite are higher resolution and precision of the aerial images, but depending on the size of the area being flown, a greater number of images might be required to survey a given area. The increase in the number of images required to survey an area increases the cost and processing time due to increased mosaicking and geocoding. Aircraft require fewer images to cover a survey area, so both processing cost and pro- cessing time are decreased, whereas satellite images require no mosaicking or geocoding but have a much lower spatial resolution. As for cost, an economic breakeven point be- tween UAVs and the other remote-sensing platforms exists between 5 and 50 hectares of area coverage; use of aircraft remains at a similar cost with satellites over a large range of survey areas. The choice of remote sensing platform de- pends on the anticipated application of the acquired data. The higher the spatial resolution of the image, the better it will represent the potential intra-vineyard variability of the sur- vey area. Accurate mapping of the intra-vine- yard variability is essential for appropriate implementation of precision viticulture prac- tices. Thus, UAVs are the best choice for remote sensing in regard to precision viticulture implementation. UAV hardware UAVs are either a fixed-wing type platform (see Figure 1a) or a multi-rotor type platform (Fig- ure 1b). The two different platforms allow for different payloads. For example, Zarco-Tejada et al. 37 used two fixed-wing UAVs, one with a 2-meter wingspan with a 5.8-kilogram payload and a UAV with a 5-m wingspan with a 13.5-kg payload. In comparison, a multi-rotor UAV platform used by Santesteban et al. 29 had a 2-kg payload, while a multi-rotor UAV platform used by Turner et al. 33 had a 1-kg payload. The payload dictates the length of flight and what sensors a UAV can carry. UAVs utilize both hardware and software to ensure that the UAV can retrieve data. The UAV hardware uses vari- ous components for flight navigation and ac- curate determination of the UAV's relative position. A flight control board consists of a pressure sensor and accelerometers to calcu- late and align the UAV in relation to gravity. 24 The navigation system, consisting of a digital compass 29 or a magnetometer 35 and a GPS module, allows a UAV to be programmed to fly autonomously and bring itself back to the operator. Aside from hardware required for flight and accurate positioning, the hardware used for measuring of canopy variables of interest must be considered. Of crucial importance, UAV plat- forms have a camera mount for attaching a sensor or multiple sensors. The UAV "Viptero" had servomotors that compensated for the pitch and roll of the UAV. 24 They also used an elastic suspension system to decouple the camera from the UAV platform to dampen the vibration caused by the UAV's rotating propellers. UAV sensors UAVs equipped with appropriate sensors can collect useful information (leaf temperature, vine water status, canopy vigor, etc.) and the Figure 2a (left) is a normalized difference vegetative index (NDVI) map produced by mosaicking all the data points from a UAV flight. The NDVI map in Figure 2b (right) was produced by extracting UAV data from spe- cific GPS locations adjacent to sentinel vines (indicated by the black dots) in the same vineyard.