A number of factors need to be understood initially.

(1) Alignment is based upon the use of light, though not on being able to see where you are going!

(2) The theory relies upon three dimensional co-ordinate geometry.

(3) The knowledge that junction error will increase with distance, together with the commercial need to keep construction time short, normally means tunnelling from the ends towards a junction in the middle.

(4) Difficulties arise because of short baselines and no intermediate fixes.

(5) Terminology needs some explanation. Thus desired alignment is designed and then set out in the tunnel. Post-construction, a wriggle diagram is prepared and final alignment computed.