Building upon the methods and findings of previous chapters, this chapter proposes a quantitative approach for describing slope geometric features. Inspired by the concept of rock joint roughness, a Slope Surface Roughness Coefficient (SSRC) is introduced to characterize the variability of slope surfaces and gradients. The method is applied to a case study of a rocky slope along National Route 251, spanning approximately 4500 m along the coastline from Unzen City to Minamishimabara City, Nagasaki, Japan. Slope gradients and surface undulations of 3D slopes were extracted using both Rhino and ArcGIS, and the SSRC values were calculated accordingly. Based on the linear relationship between SSRC and the Factor of Safety (FoS), the approach was used to assess safety factor variations for six actual slopes measuring 140 m × 140 m with heights of approximately 80 m under rainfall conditions. The predicted values closely match numerical simulation results, demonstrating that the SSRC-based method provides an efficient and accurate means of estimating 3D slope stability under rainfall. This approach not only offers a practical tool for individual slope assessment but also establishes a solid foundation for subsequent regional-scale slope stability evaluations using machine learning models, facilitating more informed decision-making in geotechnical engineering and hazard mitigation.