How did the Channel Tunnel builders navigate underground to meet in the middle?

The British and French efforts, which had been guided by laser surveying methods, met first with the completion of the undersea service (access) tunnel.

Cheers.

Channel Tunnel Boring Machine

I don't really know, i would assume through mathematics and calculations! (i think gps can work underground , though!) I know the ancient Romans would frequently construct tunnels for aqueducts through mountians by starting at both sides and meeting in the middle, it just takes a bit of math!

if joining tunnels' centre lines miss by 20cm----

somebody made a mistake

with lasers to measure distance and modern instruments---

one would expect to get better than 1cm over a distance of 80km (the swiss are good at this)

ps tunnel/mine surveyors work off the ceiling of the tunnel---

that's where they put there 'pegs' (markers)

Magic. They used a divining rod and found a line of embarkation and delineation to follow, after burying the eye of a newt and burning the eyelashes of 100 fleas.

It works every time.

The miraculous part of it was not the magic, but that you could get the French to do anything right! They usually give up and go home...

the tunnel was about 10 centimeters out on the join..the machines were kept straight by infer red lines ..i must admit it was a great feat of engineering//

One went South from London.

The other went North from Paris.

So you've got a pretty good chance that they will meet in the middle. And they did !!! ;-)