Roland Perry:
Surveying works because light travels in straight lines (or so the
Physicists tell us).

No, actually, what they tell us is that it travels in straight
lines *unless* it's traveling through a non-uniform medium.
When the Channel Tunnel was being built, they had to correct the
laser-based alignment for the variations in air temperature
within the incomplete tunnel.

However, I doubt that this would be much of an issue for the
rather shorter distances between stations on Crossrail.
--
Mark Brader | "The speed of sound is considerably less than the
Toronto | speed of light -- that is why some people appear bright
| until you hear them talk."

My text in this article is in the public domain.