The results that follow were obtained by a company that specializes in tunnel surveys and inspections by using their most accurate laser instrument. In this set-up, the laser scanner is mounted on a vehicle with its rotating axis parallel to the tunnel axis.

The vehicle must travel at a speed as constant as possible because the scanner, once started, keeps spinning at a constant speed and it cannot be stopped until the vehicle reaches the end of the area to be surveyed. Therefore, the scanner projects a spiral onto the tunnel walls, whose pitch depends on the speed of the vehicle.

The survey refers to a two-lane road tunnel in which the roadway enclosure (walls and ceiling) was completely tiled. One lane at a time was closed to traffic. The following figures compare side-by-side the crack surveys as carried out by Tonon USA and by means of the results provided by LiDAR.

  • Tile grout lines appear wobbling in the LiDAR model, but show correctly straight in Tonon USA model. As a consequence, no change detection may be provided by the LiDAR model (e.g., changes in crack length); measurements of crack lengths and spall areas in single surveys are also questionable because the model is distorted.


Tonon USA survey of roof. Notice perfect representation of square tiles.

LiDAR survey of roof. Notice the wavy representation of square tiles, which reflects a wavy 3D model of an otherwise flat roof.

  • All cracks as well as damaged and marked tiles may be easily and reliably identified in Tonon USA model, whereas this is not the case in the LiDAR model.
  • The LiDAR model included vehicles that moved in the open lane (opposite to where the laser scanner was traveling), whereas the Tonon USA provided a single model of the entire tunnel. As a consequence, the LiDAR survey requires the user to work with two separate models (one for the left wall and the left part of ceiling, and one for the right wall and the right part of ceiling); if a crack or a spall extends over the entire tunnel, it may be identified as two separate features.

LiDAR survey of the entire the tunnel


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