## 3 Results and Discussion
### Field Tests  
To verify the practical performance of the proposed path planning method, we conduct a one-week outdoor flight test. The tests will be conducted daily between 2:00 and 3:00 at noon from 2 January 2022 to 8 January 2022 at the Western Playground of Hebei University of Technology. As shown in the red zone in Figure 8, the measurement area is a space area of about 80 m \(\times\) 100 m in size and 20 to 100 m in height. There are traffic lanes around, and the buildings to the southwest are two student centenes. The four-rotor drone used is a \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Algorithm** & **L** & **Path Length** & **Time** \\ \hline LMP & & 746.94 & 523.13 \\ HGC & 3.00 & 472.85 & 428.85 \\ HGTB & & 451.29 & 358.17 \\ \hline LMP & & 563.57 & 381.18 \\ HGC & 4.00 & 367.19 & 304.99 \\ HGTB & & 352.18 & 235.05 \\ \hline LMP & & 433.28 & 287.69 \\ HGC & 5.00 & 285.79 & 197.77 \\ HGTB & & 278.33 & 169.95 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Algorithm budgets for different coverage densities in study area A.  
Figure 7: AirSim simulation system. ZD550 with a wheelbase of 550 mm and a fuselage made mainly of carbon fiber and metal, with 14-inch carbon fiber propellers. Its tripod and arm are made of hollow carbon fiber tubes, which effectively reduce its weight with the premise of ensuring hardness, with a maximum additional load of 3 kg, using a 10,000 mAh-65 power battery, and the no-load flight time is 30 min. During the test, the rotorcraft flies according to the planned path, as shown in Figure 8b. The airborne gas acquisition device collects gas data at a frequency of two times per second, and the experimental measurement is performed twice a day. A total of 14 gas data are obtained for the experiment: 2 per day. Each measurement lasts about 20 min, and the measured air pollution components are CO, PM2.5 and PM10. The average value of the measured data for each day was selected as the atmospheric pollution gas concentration on that day, and compared with the daily air quality data released by the Huaihe Road State Control Station of the Tianjin Ecological Environment Monitoring Center (about 10 km away from Hebei University of Technology), As shown in Figure 9. The solid line is the site data, and the dashed line is the experimental result of environmental measurement. Taking mean absolute error (MAE) and root mean square error (RMSE) as evaluation indicators, the smaller the value of the evaluation indicators, the smaller the error. The results are shown in Table 4. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Index** & **CO** & **PM2.5** & **PM10** \\ \hline MAE & 0.47 & 53.04 & 19.71 \\ RMSE & 0.53 & 56.38 & 24.44 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of evaluation index results. Figure 8: (a) Field tests. The red area is the detection area. (b) 3D detection path. (c) UAV in field tests. On the time scale, the results of the pollution gas concentration measured in this experiment are basically consistent with the monitoring results of the Huaihe Road State Control Station. Since there is a certain distance between Hebei University of Technology and the Huaihe Road State Control Station, and the real-time environmental conditions have deviations, the deviation between the experimental results and the results of the State Control Station is within an acceptable range. We randomly selected one day's experimental data, used the Kriging interpolation method to interpolate the gas data, and analyzed the distribution of polluted gases in the measurement space. The results of PM2.5 interpolation are shown in Figure 10. It can be seen from the interpolation results that the concentration of pollutants on the line from the southwest to the northeast of the detection space is higher than that of other areas, and the concentration gradually decreases with the increase in height and longitude. This may be the result of the spread of pollutants from road vehicles and canteens in the southwest of the detection area to the northeast.