# Automatic Extraction of High-Voltage Power Transmission Objects from UAV Lidar Point Clouds
## 3 Method
### Reconstruction of Power Transmission Components
#### 3.5.2 Modeling of pylons Based on Tower Center Localization  
In the maintenance of power transmission, special attention should be paid to the stability of the HV pylons at a large scale and the accessories on them, i.e., the insulators and signs of tower number. Generally, the geographical coordinates of each pylon are required when dealing with the security of the transmission target. The main difference in this reconstruction approach is that we reconstructed not only the pylons, but also the insulators on them. First, the accurate planar coordinates of the pylons were calculated. Next, the library of tower templates was built according to the four basic types. Thus, the position error of the models built of each object in the corridor scene was smaller than others' work. Moreover, the pylons were reconstructed as a whole model and were detailed by locating and modeling their insulators. Figure 9: 3D reconstruction of PLs in a span: (**a**) fitted PLs within a span; (**b**) fitted PLs with two adjacent pylons (rendering by electrical phase). **Planar Center Localization:** Having identified the grids where the power towers are located, their structure and shape were abstracted to the four categories in Figure 10. Their distribution varied, which had an obvious influence on the plain projection feature of density and feature of height distribution. When calculating their central points, multiple voting mechanisms were used to search for the most reliable point centers of the horizontal section in multiple iterations. As HV towers are regular to some extent, a horizontally sliced layer at any height of a tower body has an in-plane symmetry, as long as the points on the tower are not largely missing. The centers of those sliced layers based on the stratification in vertical direction can be used to calculate the accurate planar coordinates of the HV tower. As shown in Figure 11a, there were still some vegetation points attached to the bottom of the tower, and the tower points were incomplete at the top. The central positions of the sliced layers shown in Figure 11b were also affected by the missing points. According to the verticality of the HV tower, there should have been a small angle between the vertical direction and the vectors formed by the centers of the layers (Figure 11c). The six points in the middle of Figure 11d describe the identified candidate central points, analyzed statistically by the algorithm shown in Table 1, in which \(\mathrm{M_{1}^{c}}\) is a collection of tower grid, which is then divided in to several layers \(\mathrm{C_{i}}\) 2 meters in height; \(\mathrm{d_{rc}}\) is the horizontal distance of two center points; \(\mathrm{\theta_{rc}}\) is the angle between the z-axis and the line through the two central points determined by center point pairs, consecutively; \(\mathrm{d_{Mrc}}\) is the maximum of collection \(\mathrm{[d_{rc}]}\); and \(\mathrm{p_{r}}\) and \(\mathrm{p_{c}}\) are center points of the two layers corresponding to \(\mathrm{d_{Mrc}}\). Figure 11: Illustration of HV tower center localization: (**a**) The point cloud of an incomplete HV transmission tower; (**b**) Center points by layers; (**c**) The procedure of center point selection; (**d**) Identified center points. Figure 10: A schematic diagram of HV tower: (**a**) drum-like tower; (**b**) goblet-like tower; (**c**) tower in a zigzag structure; (**d**) cat-head-like tower. **Pylon modeling:** In this paper, the methodology proposed by [PERSON] et al. [PERSON] et al. (2018) was applied to build the 3D pylon models, based on model fitting using a library of high-precision tower models built manually according to relevant specifications. The 3D model was initiated at the center of the detected tower, defined by the planar location and half of its height. The model size was determined by the voltage level when the power transmission was designed, as shown in Figure 12a. The Gibbs energy model was established to estimate point cloud and model fitness during the reconstruction process.