## 6 Conclusions  
In this study, a high-precision and high-resolution DEM generated using airborne-based LiDAR survey and mapping technique and related value-added maps were used to understand the topographic and geographic features in the vicinity of Suao to Dongao in northeastern Taiwan. A field investigation was carried out and data were obtained following the excavation of the Dongao Tunnel to yield geological information about the study area. Interpretations were compared with on-site geological observations to assess the use of airborne-based LiDAR DEM in geological survey and mapping. The following conclusions were drawn.  
1. The airborne-based LiDAR DEM captures more precisely the topographic and geographic features of strata of the study area than do topographic maps produced using traditional aerial photogrammetry. The precise locations of geological structures, such as faults and lineaments, as well as the drainage system, can be identified accordingly. On clinometric and hillshade maps, different lithologies are distinct, as is the drainage system. Such maps therefore help in the finding of outcrops for conventional surface geological investigation.
2. High-resolution airborne-based LiDAR DEM for identifying lithological boundaries, geological structures such as faults and lineament, and drainage systems, can be combined with on-site and underground excavation-based verification, to refine geological surveying and mapping.
3. A scarp on the surface of a slope is an evidence of the deformation of the underground rock mass. However, the scope in which the rock mass has a high degree of weathering and the attitude of the tilted discontinuity may exceed the scope in which scarps on a slope are identified in a clinometric map.
4. A field investigation by a professional geologist is still the most reliable method of geological surveying and mapping for the purpose of determining the engineering significance of geological features, their quantitative characteristics and corresponding parameters. Combining information thus obtained with that inferred from a high-resolution DEM can provide a more comprehensive understanding of the geological survey, including lithological distribution, geological structure, and locations of potential instabilities of a slope, favoring the assessment of tunnel engineering risks, especially in the initial stages of engineering projects, such as a feasibility study, planning, and design.  
Conceptualization and methodology, P.-C.L. and T.-T.W.; investigation, P.-C.L.; writing--original draft preparation, P.-C.L. and T.-T.W.; writing--review and editing, W.L., T.-T.W., and Y.-C.H.; supervision, T.-T.W. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors would like to thank the Fourth District Maintenance Construction Office and Suhua Improvement Engineering Office, Directorate General of Highways, Ministry of Transportation and Communications, Taiwan, for supporting high resolution DEMs for subsequent interpretation. Valuable comments from [PERSON], [PERSON]; kind supports of data processing by [PERSON], [PERSON]; and field investigation by [PERSON], [PERSON], are also acknowledged.  
The authors declare no conflict of interest.