# Hybrid Sensing Platform for IoT-Based Precision Agriculture
## 4 Results
### Data Calibration
#### 4.2.3 Aerial Spectrometer Data Calibration Using Ground-Based Data--Experiment 6  
In the previous experiment, the impact of environmental variables on the infrared optical sensor was discussed. In this experiment, the impact of the same variables on the aerial spectrometer sensor is analysed. As discussed in the validation section, in this research, we focus on three spectrometer channels, which are channel E, which represents the green colour; channel H, which represents the yellow colour, and channel O, which represents NIR. Table 8 shows these channels' correlations in the morning, afternoon, and evening data collection cycles. As the data in Table 8 show, the aerial spectrometer readings are significantly more accurate during morning cycles compared with afternoon and evening cycles. On the other hand, at afternoon cycles, the aerial spectrometer readings show an inverse correlation where the correlation coefficient value is negative. This shows that the solar radiation value remarkably impacts the aerial spectrometer readings. In order to analyse the impact of solar radiation and overcast on spectrometer readings, the data from cycle 9, which were taken in the afternoon and include both sunny and cloudy weather, are analysed. In this cycle, the aerial data were collected mainly from the target plant. After analyzing the aerial video in this cycle, it was noticed that there were 23 data points where the UAV was positioned directly on top of the target plant at close range. These data can be used to analyse the aerial and ground-based spectrometer readings when both aerial and ground-based sensors are reading the data from the target plant. One important issue regarding this cycle is that, from data point 10 to 16, the weather turned cloudy and there was no direct sunshine. This can be useful to analyse the impact of overcast and solar radiation on the spectrometer data. As Figure 16 shows, during the points were there was overcast, the distances between aerial and ground-based spectrometer readings were significantly reduced. This shows that the high fluctuation in aerial spectrometer readings is due to direct sunshine and high solar radiation. The wind speed and wind gust in this cycle were 17.2 and 28.3 km/h, respectively. This shows that despite the relatively high wind speed and wind gust, the spectrometer data were remarkably improved during cloudy weather. Therefore, it can be concluded that the main reason for aerial spectrometer readings' fluctuations is direct sunlight and high solar radiation. Table 8 shows that the aerial spectrometer readings improve during morning cycles, where there is less solar radiation. In order to compare the aerial data's accuracy with ground-based data, the morning data collection cycles' videos were analysed to identify the data points where the UAV collected data from the target plant from close range. In total, 37 data points were identified, which are presented in Figure 17. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Cycles**} & **Solar** & **Wind Speed** & **Wind Gust** & **Aerial Channels** & **Aerial Channels** & **Regression with Ground-Based Channels R\({}^{2}\)** \\  & **Average w/m\({}^{2}\)** & **Average km/h** & **Average km/h** & **Ground-Based Channels** & **Ground-Based Channels R\({}^{2}\)** \\ \hline All & 357.1 & 13.3 & 20.5 & E: 0.75, H: 0.68, O: 0.72 & E: 0.56, H: 0.46, O: 0.52 \\ Morning & 74.3 & 9.1 & 14.1 & E: 0.75, H: 0.76, O: 0.74 & E: 0.56, H: 0.57, O: 0.55 \\ Afternoon & 757.7 & 14.9 & 21.3 & E: \(-\)0.25, H: \(-\)0.35, O: \(-\)0.28 & E: 0.06, H: 0.12, O: 0.08 \\ Evening & 252.1 & 15.2 & 24.7 & E: 0.37, H: 0.41, O: 0.38 & E: 0.13 H: 0.16, O: 0.14 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Aerial and ground-based spectrometer data comparison. based on different time cycles and observation context variables. As can be noticed from Figure 17, the distance between aerial and ground-based readings, particularly in the NIR channel, is high.