## 3 Results
### Accuracy and Flight Altitude Correlation  
There is a conflict between the UAV resolution and image range. On the premise that camera parameters remain unchanged, the wider capture area, the fewer pixels are distributed over a certain area. Hence, the UAV flight altitude is an essential factor to the accuracy of displacement analysis and clarifying the correlation between accuracy and flight altitude helps to control the accuracy of this proposed method, meanwhile, further guarantee the validation of the result.  
Several experiments were carried out in on-site researches. In the experiments, there were three factors taken into consideration, including flight altitude, monitoring point area and the measurement error comparing with actual displacement based on the T = total station or RTK survey. Because the shape of the monitoring points are always sets as square or circle, the side length or radius are used in analysis instead of the area.  
In the field study, monitoring points and datum points need to be set up (Figure 18), red square were used as monitoring point and blue square were used as datum point. The datum point is used to calibrate coordinate system of point clouds by UAV and the coordinate of datum point was measured by a RTK survey. The calibrate process is carried out by marking datum point in every image. The monitoring point needs to be measured to obtain its own displacement. In the process of measurement, the monitoring point will be identified by set the value color H and S of HSL color space.  
The flight heights of UAV were set at different elevations such as 20 m, 30 m, 40 m, 50 m, 60 m, 70 m, 80 m, 90 m, 100 m, 110 m and 120 m respectively. The side lengths of the selected monitoring points are 15 cm, 20 cm, 30 cm, 40 cm and 45 cm, respectively. Controlling the error within 10 mm, the correlation between flight altitude and monitoring point size is shown in Figure 19a, it is easy to conclude an approximate linear correlation that with the flight altitude rises, the size needs to be set larger. In Figure 19b, when controlling the flight altitude as a constant (30 m) to analyze the correlation between monitoring point size and error that draw a conclusion: the larger the size of the monitoring point, the smaller the error. When controlling the size of monitoring point (40 cm), the correlation of the UAV flight altitude and error is shown in Figure 19c.  
Figure 17: The location of the study area and scene of site slope.  
It is exactly obvious that there are some results can be concluded as follow: (1) Approximate linear correlation between monitoring point size and UAV flight altitude can be presented as an equation \(y=0.33x+9.24\). It shows that to make the error meet the requirements of construction project and specification, the monitoring point size needs to be set larger when the UAV altitude rising. (2) Monitoring point is better from 30 cm to 45 cm, accordingly the measurement error will be reduced with the size increases in a certain extent. (3) In order to ensure the validation of this method, when the size of monitoring point is set to 40 cm, it is recommended that the UVA altitude of less than 60 m. While the altitude excess 60 m, the measurement error will increase greatly and quickly. However, in the application, the relative height cannot be strictly controlled.