## 6 Conclusions  
In this paper, a laser rangefinder is applied to obtain the distance and rotation angle between an image plane and target, and the unit pixel size is calculated by the proposed geometric transformation formula. The U-net segmentation algorithm is used to extract crack areas in images, and the Canny method is applied to refine the edges for mathematical standards. Both systematic and algorithmic errors are tested by standard objects, and the system is applied on several concrete walls to verify the effect of photographing angle and distance on measurement accuracy. The measurement results indicate that the proposed system can measure a crack in a non-vertical position, and the proposed algorithm can extract a crack from different background images. The following are the main conclusions from this study:  
Figure 27: The interference of the protrusion deep in the crack on the accuracy of dam crack segmentation. (**a**) segmentation discontinuity, red circle is the protrusion position; (**b**) segmentation error, red circle is the different protrusion style; (**c**) missing part of crack segmentation, red circle is the continuous protrusion; (**d**) protrusion similar to concrete surface, red circle is the protrusion position.  
1. The measurement results for standard artificial cracks prove that the capturing angle and distance can significantly impact the accuracy; since the number of pixels the target occupies is reduced, the image resolution and unit pixel size would affect measurement results.
2. To further analyze the influencing factors of measurement accuracy, a concrete wall in the lab is adopted to measure the crack width. With capturing distances and horizontal angle changing from 5 to 20 m and \(-65^{\circ}\) to \(50^{\circ}\), 12 cracks widths are measured, and the average absolute error is less than 0.2 mm, which proves that the crack images taken from different angles and distances can be used to accurately calculate the crack width.
3. The performances of crack extraction with different backgrounds are also analyzed on several concrete walls. The maximum error occurs at the furthest position, which is calculated as \(-0.15\) mm measured from 15.647 m. For cracks with a small captured angle, the relative error is less than 5%, which can prove the accuracy of the segmentation algorithm.
4. The measurement results on the concrete dam show that the protrusions that occur deep in the crack can affect the segmentation results, and the measurement error is increased. The measurement results are accurate and robust, which means the method presented in this paper has practical and scientific novelty.
5. In general, for the equipment used in this paper, the capturing angle should not be greater than \(50^{\circ}\), and the photographing distance should be less than 30 m. The maximum measurement error obtained in this way is less than 0.3 mm. For concrete crack measurements, the performance of the camera and laser rangefinder combination system based on the improved U-net algorithm and Canny method is accurate and stable.  
Conceptualization, J.L. and F.K.; methodology, S.Z.; validation, F.K.; formal analysis, J.L. and F.K.; writing--review and editing, S.Z. and F.K. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Key R & D Program of China grant number 2016 YFC0401600 and 2017 YFC0404900; National Natural Science Foundation of China grant number 51779035, 51769033 and 51979027, 52079022; Fundamental Research Funds for the Central Universities grant number DUT21 TD106.  
Not applicable.  
The authors declare no conflict of interest.