Fig. 1. Aerial images and diagrams of towers. (a) Front image; (b) front side image; (c) side image; (d) front view diagram; (e) front side view diagram; (f) side view diagram

Fig. 2. Broken lines formed when wire passing through straight tower

Fig. 3. Location relationship between guiding lines and wires in tensile tower

Fig. 4. Structural characteristics of tower. (a) Cross slanted segments of tower head; (b)(c) slanted segments converged onto main frame of tower body; (d)(e) crossing characteristics in tower

Fig. 5. Framework of tower detection from far to near

Fig. 6. Calculation of gradient for each pixel point

Fig. 7. Angular distribution of gradient

Fig. 8. Gradient statistical histogram in each cell

Fig. 9. HOG feature extraction and MLP perception model classification

Fig. 10. Visual perception platform for UAV and local areas of tower under different orientations. (a) Visual perception platform for UAV; (b1) front of tower top; (b2) front side of tower top; (b3) side of tower top; (c1) front of tower body; (c2) front side of tower body; (c3) side of tower body; (d1) front of tower bottom; (d2) front side of tower bottom; (d3) side of tower bottom

Table 1. Accuracy rate of identification of local area orientation of tower

Table 2. Time required for identification of local area orientation of towerms

Table 3. Accuracy rate of local area orientation of tower after data augmentation

Table 4. Time required for local area orientation of tower after data augmentationms