# Automatic Extraction of High-Voltage Power Transmission Objects from UAV Lidar Point Clouds
## 3 Method
### Segmentation of Power Lines and Pylons
#### 3.3.2 Regional Segmentation of Candidate High-Voltage Towers Based on 2D Features  
HV pylons, which support the transmission lines, sit on the ground, as shown in the point cloud data. Therefore, they show a continuous distribution in the vertical direction. Furthermore, according to the regulations governing overhead transmission lines, they must maintain a safe clearance to the surroundings or ground objects, so HV pylons are generally built more than 20 meters high. Figure 6 illustrates the characteristic of large local height difference in the areas where the HV towers are located. In the horizontal direction, the wing length of the HV pylons is related to the corresponding voltage level of the PLs. Given the voltage level, the distribution range of pylon points on the horizontal projection plane should be finite, as in Equation (4).  
\[\mathrm{R}_{(m,\,n)}\leq(\mathrm{r}+5), \tag{4}\]  
where \(\mathrm{R}_{(m,\,n)}\) is the radius of the pylon candidates in the projected ground plane and \(\mathrm{r}\) is the wing length. These distribution features of HV pylons were utilized to divide the candidate pylon areas.  
The segmentation was composed of five steps:  
* The 2D grids of large height difference in the input data were first identified according to the height difference feature. As seen in the red dashed boxes in Figure 6e, these grids mainly consisted of towers, individual tall trees, or groups of clumped trees.
* A 2 \(\times\) 2m grid was created as a moving window to detect the finer characteristics of the grids with large height differences. The window was initialized with the same features as the grid. When it moved, its features were updated. The height difference value of the moving window that was greater than the pre-defined threshold (the minimum height difference between PLs and the ground from previous analysis) was retrieved and the corresponding grid was marked.
* The vertical continuity of the points in each marked grid was checked. Given the maximum height gap threshold (usually set as the absolute height of the tower), grids containing a window of a higher gap than the threshold were removed to filter out outliers.
* The remaining grids were clustered to group the grids belonging to the same tower. Neighboring grids were clustered when their height differences were greater than the pre-defined threshold.
* The distribution range of HV pylons in the planar projection was utilized to differentiate them from other clustered objects. The principal direction of each cluster was calculated. The semi-major axis of a tower in the planar projection range in the principle direction was not greater than the length of HV tower's wing. From the horizontal projection range described by Equation (4), blocks of clumped trees were filtered out because their distribution ranges in the planar projection failed to meet the equation in the horizontal plane. Small clusters (see the dashed box in Figure 6f) were  
Figure 6: Illustration of the candidate HV tower extraction process: (**a**) the overall view of the data without PL points; (**b**) partial view of the towers in (a); (**c**) detailed view of a tower in (b); (**d**) a sub-dataset to be processed with PL points removed; (**e**) grids with big height differences; (**f**) results of grid clustering colored by height.  
then excluded by judging whether each grid in the clustering blocks had an extremely local high value in the DSM feature map.  
Figure 6a shows one dataset in which PL points were removed, and Figure 6b-d shows the top view and side view of some pylon areas. As a result of the extraction of pylons, points on the majority of non-power transmission components and tall clumped trees and arbors were removed. As can be seen from the result in Figure 6f, by extracting the grids with large height differences, most non-tower points were removed and the target tower was completely retained.