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@stevo_com
they uise the same things on the underground. Fixed surveying prisms and predetermined points. They instal a total station (fancy theodolite) that fires a laser at roughly where the prisms last were. The 45 degree angled mirrors in the prism mean the total station can find the exact centre, it will mark that in 3D space and move onto the next one. Someone will come along, collect the total station, look at the results and then decide if we're all about to fall into hades.
You can sometime spot them up on their little ledges above the platforms and escalators.