## 6 Conclusions  
The geological survey of hydraulic engineering always suffers from ineffective data acquisition and backward field analysis and data management methods. In this research, three essential techniques for the digitization of the fieldwork of hydraulic engineering are proposed, namely, digital data recording, interpretations for 2D images, and computer-aided analysis for geological phenomena. The specific methods include (1) modelling for 3D real scenes, (2) structured storage for geological data, (3) interpreting from 2D images to 3D information, (4) evaluation of accumulation body, (5) block instability analysis, and (6) intelligent analysis for geological data.  
To implement the above solution, we presented: (1) an oblique photography-based scheme for displaying the real field scene, (2) a hierarchical model for data storage, (3) a control point-based interpreting method for extracting 3D information from 2D pictures, and (4) a Delaunay triangulation-based method for the evaluation of accumulation bodies. Additionally, we chose the polar stereographic projection method for the instability analysis of slopes, and built a deep neural network for the intelligent recognition of rocks. As a result, all the above methods together with the techniques of GIS, GPS, and 3D modelling, were integrated into a digital tool.  
The tool was also applied to a case study involving hydraulic engineering where information was recorded along the river that may negatively influence the dam. Based on the recorded geological information, a preliminary 3D geological analysis was performed to estimate the bottom surface of the accumulation bodies. In addition, it enables 2D images to be interpreted into 3D information that can be used for geological structural analysis. A comparison was made between the output results and the stitched photos, which proved that the method was influential in the case study.  
In addition, it should be noted that the tool is for generalized and preliminary surveys, and aims at helping engineers acquire, manage, and analyze geological information in a highly efficient way during their field work. Therefore, the modules integrated in it are not based on complex calculations. For example, the presented estimation method for accumulation bodies needs engineers to set a center depth by experience. Technically speaking, this method is not rigorous enough. However, the exact depth of an accumulation body is challenging to determine in the field, but can only be calculated through extensive geological evidence and repeated trial-calculation in the lab. Therefore, the proposed method is to provide a reference for engineers for subsequent fieldwork. With the development of computer techniques and survey methods, the proposed solution and tool will be continuously improved to enhance the rigor of methods and the usability of the functions.  
Methodology and writing--original draft preparation, W.Z.; writing--review and editing, S.H.; data curation and project administration, Y.C.; software, Y.G.; visualization, M.L. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Tianjin Science Foundation for Distinguished Young Scientists of China, grant no. 17 JCJQJC44000.  
Not applicable.  
We gratefully acknowledge the valuable suggestions of the editor and the four reviewers.  
The authors declare no conflict of interest.