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surface.
This method was developed in cooperation with Artescan-3D
Scanning, a Portuguese company that owns a Riegl VMX-250-
CS6 mobile laser scanning system (figure 2). This system has
been applied in long tunnel surveying, both motorway and
railway (Boavida et al., 2012). These can be as-built surveys of
tunnels, or can be made with the purpose of monitoring and
maintenance. Many of these surveys span along distances of
several kilometres, producing very large amounts of data. The
availability of methodologies that automatically produce
rectified, planar images of the tunnels, was found an essential
tool for analysis of these data.
Figure 2. Riegl VMX-250-CS6 mobile laser scanning system in
operation in a tunnel survey.
The method was tested with data of railway tunnels, first in
small samples of the underground of the city of Porto, and then
with data collected in the survey of a 25 km long high-speed
railway tunnel (Pajares tunnel in Asturias, Spain). Figure 3
shows a profile of the terrain and the tunnel in a distance of
more than 1 km. Tunnel slope is approximately 1.7 %.
1200
1000
Terrain
T 800 mE
= Tunnel
=
> 600
X
400
0 200 400 600 800 1000
Distance (m)
Figure 3. Profile of the terrain and the tunnel in Asturias.
2. DESCRIPTION OF THE ALGORITHM
The goal of this algorithm is to project a 3D tunnel surface into
a plan, or in other words cut the tunnel longitudinally and
unfold it to make it flat. The input is a point cloud acquired by
laser scanning inside a tunnel.
The algorithm was programmed in PostgreSQL/PostGIS,
making use of some of its capabilities of spatial search and
analysis.
The algorithm is composed by the following 5 stages.
STAGE 1
The first stage is to recalculate the tunnel axis. The original axis
can be acquired in different ways. It can be extracted from the
vehicle trajectory (obtained by the inertial navigation system);
109
from a traverse or other survey network; from a manual
procedure or from a semi automatic process involving a
skeletonization algorithm. Once a planar representation of the
axis is available, a set of points, with constant spacing, will be
generated along the axis (figure 4). This spacing (e.g. 10
meters) is a parameter defined by the user.
" 3 in M Moters
fear}
Figure 4. Tunnel axis with the original points (larger squares)
and the equally spaced points (small dots).
For each one of these points a line is created, locally
perpendicular to the axis. The direction of the axis in a point is
considered to be the direction defined by the previous point and
the next point, as shown in Figure 5.
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A. —
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ry gai
ee
pp
0 5 18 26 Meters
Figure 5. The perpendicular line (green line) at point 3 is
calculated perpendicularly to the direction defined (blue line) by
the previous (2) and the next point (4).
Now a buffer around this perpendicular line is created, with
parameters specified by the user. Then, all the points from the
point cloud that are inside this buffer are averaged to calculate
the central point of the tunnel at the respective location (the
central point is a 3D point calculated with the average values of
the three coordinates of the selected points from the point
cloud). The final 3D tunnel axis is composed by all this 3D
points. This process can be recursively repeated until the
differences between the old and the new axis are negligible.
STAGE 2
The second stage is to rectify the tunnel axis that was
automatically created. It is possible that the axis calculated in
the previous stage has errors or unwanted points. In these cases,
it is crucial to examine the result and make the desired changes
manually, before continuing to the next stage. This step is
essentially a visual inspection and possibly a manual adjustment
of few points. It is not time consuming and may increase the
results quality in irregular parts of the tunnel.
STAGE 3
The third stage is the segmentation of the point cloud. For each
vertex of the axis a plan perpendicular to the trajectory is
calculated (direction defined as before, between the previous
point and the next point). These plans can be materialized with
their dimension defined by user (Figure 6).
en n
Figure 6. The tunnel axis (red line) is cut by perpendicular plans
(green plans) crossing the axis in its vertices, segmenting it.
The tunnel axis is cut in n — 1 segments (n is the number of
axis vertices). For each segment, comprehended between two
