Metro train tunnel construction. Surveyors using laser levels (mounted on tripods) to monitor the alignment of a tunnel and make sure it is not being excavated at an incorrect angle or direction. This construction work took place between 2000 and 2005, on the Paris Metro, the underground train system in Paris, France. The Paris Metro was founded in 1900, and by 2005 had over 200 kilometres of tunnels and over 300 stations.
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