Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 150

measuring displacement (for example optical) have become established. This is indeed
less precise, but does not cause any obstruction at the face and can also cover a greater
number of survey points permanently in less time [24].
4.3.1.3 Optical surveying of displacement with electronic total station
Using the digital surveying instruments and powerful computer programs for data processing
that are now available, displacements in three dimensions of any survey points can be deter-
mined in absolute coordinates, which means a reference system more stable than the tunnel.
This makes it considerably easier to detect local and large-scale deformation than with relative
surveying methods like convergence measurement or levelling. A three-dimensional display of
the results can enable comparison of the displacements of all the survey targets in a selected
area. One essential feature of optical surveying of displacements is the free positioning of the
survey station (Fig. 4-13). The position can be chosen flexibly to provide the best sight lines
and the least obstruction of construction work. The accuracy is determined essentially by the
geometrical relationships and the precision of the surveying instrument and the targets.
Figure 4-13 Free posi-
tioning of the total station
for optical 3D displace-
ment surveying [136].
Figure 4-14 Surveying of
the absolute position of
convergence targets with
display of their movement
vectors [187].