494
F. Bordry et al.
The first level of this network is a surface network. It is constituted of monuments
solidly anchored to the earth by means of concrete works, forming a very welldefined basic framework and from which the links to national and international
reference system can be established. It will also be used for regular checks
and eventual extension of the project. The determination of the co-ordinates of
these monuments is done by very accurate triangulation, trilateration, leveling
measurements and nowadays when possible by Global Navigation Satellite System
(GNSS) measurements. The accuracy of these network points has to be in the
range of a few mm (1σ). It could be advisable to equip some network points with
permanent GNSS receivers in order to have their position permanently recalculated
together with permanent GNSS stations located in the vicinity of the site.
This surface network is transferred to an underground network by the means of
several techniques among which one can mention:
• Angles and distances measurements with total stations
• Azimuthal orientation with gyro measurements
• Plumb lines measured by theodolites at the top and at the bottom of the shaft at
the same time
• Nadiro-zenithal telescope
• Calibrated Electronic Distance Measurers (EDM) and optical levels for the
altitude determination.
All the observations are processed together as a spatial block. Deflections of the
vertical and meridians convergence are also taken into account. For a pit as deep as
140 m, the accuracy is estimated to be in the range of one mm in the XY plane and
better than 2 mm in H (altitude), both values given at 1σ.
The underground network is constituted of tripods regularly spaced out along
the accelerator tunnel, the distance between them being often directly linked to
the lattice of the machine. The topographical traverse linking one access shaft
to the adjacent one is realized by gyro-theodolites, theodolites and very accurate
Electronic Distance Measurer, namely the Mekometer. Offset distances with respect
to a nylon stretched wire are also frequently measured in order to improve the
“smoothness” of the network. As an example, for a distance of 3.3 km between
two points transferred from the surface network, as is the case for the LHC, the
transverse deviation is estimated to be 4 mm (1σ) in the middle between these two
points (Fig. 8.93).
In the vertical direction, the determination of the altitude of the points of the
underground network is done by leveling measurements using optical or digital
levels. The accuracy is of the order of 0.4 mm (1σ) per km. In order to have a stable
reference plane, for calculation and future stability comparison, it is advisable to
anchor references at a depth of 25 to 30 m under the tunnel level in stable rock. Two
of these references installed in the SPS have given proof of their utility and therefore
such a reference has been installed in the vicinity of each of the eight LHC shafts.
F. Bordry et al.
The first level of this network is a surface network. It is constituted of monuments
solidly anchored to the earth by means of concrete works, forming a very welldefined basic framework and from which the links to national and international
reference system can be established. It will also be used for regular checks
and eventual extension of the project. The determination of the co-ordinates of
these monuments is done by very accurate triangulation, trilateration, leveling
measurements and nowadays when possible by Global Navigation Satellite System
(GNSS) measurements. The accuracy of these network points has to be in the
range of a few mm (1σ). It could be advisable to equip some network points with
permanent GNSS receivers in order to have their position permanently recalculated
together with permanent GNSS stations located in the vicinity of the site.
This surface network is transferred to an underground network by the means of
several techniques among which one can mention:
• Angles and distances measurements with total stations
• Azimuthal orientation with gyro measurements
• Plumb lines measured by theodolites at the top and at the bottom of the shaft at
the same time
• Nadiro-zenithal telescope
• Calibrated Electronic Distance Measurers (EDM) and optical levels for the
altitude determination.
All the observations are processed together as a spatial block. Deflections of the
vertical and meridians convergence are also taken into account. For a pit as deep as
140 m, the accuracy is estimated to be in the range of one mm in the XY plane and
better than 2 mm in H (altitude), both values given at 1σ.
The underground network is constituted of tripods regularly spaced out along
the accelerator tunnel, the distance between them being often directly linked to
the lattice of the machine. The topographical traverse linking one access shaft
to the adjacent one is realized by gyro-theodolites, theodolites and very accurate
Electronic Distance Measurer, namely the Mekometer. Offset distances with respect
to a nylon stretched wire are also frequently measured in order to improve the
“smoothness” of the network. As an example, for a distance of 3.3 km between
two points transferred from the surface network, as is the case for the LHC, the
transverse deviation is estimated to be 4 mm (1σ) in the middle between these two
points (Fig. 8.93).
In the vertical direction, the determination of the altitude of the points of the
underground network is done by leveling measurements using optical or digital
levels. The accuracy is of the order of 0.4 mm (1σ) per km. In order to have a stable
reference plane, for calculation and future stability comparison, it is advisable to
anchor references at a depth of 25 to 30 m under the tunnel level in stable rock. Two
of these references installed in the SPS have given proof of their utility and therefore
such a reference has been installed in the vicinity of each of the eight LHC shafts.