Optimizing profile line interval for enhanced accuracy in rock joint morphology and shear strength assessments

2D profile lines play a critical role in cost-effectively evaluating rock joint properties and shear strength. However, the interval (ΔI_L) between these lines significantly impacts roughness and shear strength assessments. A detailed study of 45 joint samples using four statistical measures across 500 different ΔI_L values identified a clear line interval effect with two stages: stable and fluctuation-discrete. Further statistical analysis showed a linear relationship between the error bounds of four parameters, shear strength evaluation, and their corresponding maximum ΔI_L values, where the gradient k of this linear relationship was influenced by the basic friction angle and normal stress. Accounting for these factors, lower-limit linear models were employed to determine the optimal ΔI_L values that met error tolerances (1%–10%) for all metrics and shear strength. The study also explored the consistent size effect on joints regardless of ΔI_L changes, revealing three types of size effects based on morphological heterogeneity. Notably, larger joints required generally higher ΔI_L to maintain the predefined error limits, suggesting an increased interval for large joint analyses. Consequently, this research provides a basis for determining the optimal ΔI_L, improving accuracy in 2D profile line assessments of joint characteristics.