## 3 RTI Data Acquisition Methods
### Visual Methods  
Visual assessment is the longest-established NDI method for RTI and is still widely adopted today, particularly across the UK and Chinese rail networks [12; 13]. We subdivide methods into two classes: (i) Traditional and (ii) Modernised. Traditional evaluation is exclusively based on engineers' learnt association between visual indicators (e.g., work-manship inconsistencies, material fatigue hallmarks) and fault likelihood. Problematically, engineers infrequently share similar extents of practical experience, resulting in high subjectivity. Crosschecks and multi-pass surveys can partially reduce accuracy and consistency variations but take significantly longer to implement at increased resource cost, closure times and rail worker risk. Handwritten notetaking ambiguity, incompleteness and inherent susceptibility to human error also present issues for later analysis. They entice misinterpretation, causing unnecessary delays and disruption. However, being low expense and reasonably accurate (if performed by more experienced engineers), coupled with human-aptitude at informed predications from non-structural information (e.g., history of construction practices); traditional methods can time-efficiently localise visibly degraded quadrants requiring repair.  
Modernised methods mostly utilise Close-Range Photogrammetry (CRP) to provide referenceable intrados imagery. Units commonly employ RGB optical cameras mounted on moving platforms for stability and time-efficiency. These include: Pushcarts/Rail-Trolley (RT), Road-Rail Vehicle (RRV) and Robotic Traction Unit (RTU). Merging resultant overlapping orthophotos via mosaicing [14; 15] allows tunnels to be 'unwrapped'--permitting analysis in 2D--although we more frequently find studies adopt 3D CRP topography model reconstruction via 'Structure from Motion' (SfM) algorithms [16; 17; 18; 19].  
A noteworthy recent innovation includes 'Digital Imaging for Condition Asset Monitoring System' (DIFCAM) [20]; an RRV-mounted optical array designed to reduce crew sizes and inspection durations. Although 2014 marks DIFCAM's last major study [21], scope of its successor project DIFCAM Evolution [22] discusses subsurface imaging and automated defect recognition technology integration. However, due Visual assessment is the longest-established NDI method for RTI and is still widely a lack of available details or recent publication activity, we reside this to speculation only. Of comparable interest, [23] presents a 'Moving Tunnel Profile Measurement' system (MTPM-1) which deploys a novel rotating camera for CRP that tracks a translating laser target to achieve swift 3D capture of a 100 m tunnel in 3 min. Use of a more lightweight camera is necessary for smoother rotation and reduction of prevalent lens-distortion.  
Overall, visual methods provide extensive surface inspection prospects but are impractical for subsurface inspection since tunnel intrados' are opaque, except where defects have already exposed the subsurface. For a summary of defect types see Section 6.1.2 and consult 'Ring Separation and Debonding'. We believe proposed revisions of MTPM-1 show promise and would further benefit from fusion with automatous RTU locomotion described in [24] to facilitate 24/7 remote deployment.