How did bridge builders of old ensure both ends of the bridge would perfect meet in the middle before laser measuring was around?
In: Technology
Guide poles and ropes. You set up either poles or ropes on either side of the space you want your bridge that stretch the distance you need to cover, and then you start building between the poles/ropes. You can remove the poles/ropes after completion, but a lot of times guide poles would remain as part of the structure.
Most bridges were smaller, and often they didn’t start building from both sides, anyway.
Before lasers we used (and still do use) optical systems instead – in particular the theodolite.
A theodolite is a device that measures angles – you sight down a small scope, and by using a few readouts on the theodolite determine the direction the scope is aimed, and the angle of elevation.
If you have two known points (such as two starting points that you have positioned a set distance apart from each other), you can then use trigonometry to create a triangle between the two known points and the third you are measuring to and determine how far away
Do this accurately enough and you can plot out all the locations and heights you want, allowing you to set out your structure.
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