I have been trying for years to put together a robust system for measuring deflections of bridges. For short spans, low level, over land, it is easy. Big spans over water are much more difficult and the "modern" way is to use total stations, but can they follow live loads.
Years ago I saw a system designed by NPL and deployed on the Glen Devon Dam. It uses laser light but does not depend on the accurate pointing of the laser. The system looks like extrapolation, using a light lever, but is actually closer to interpolation (ie it is VERY robust).
Set it up ona bridge on Wednesday. Watch the deflections on the screen on YouTube.
We are looking for diurnal movement so the screen image is being captured by a Canon G9 reprogrammed using CHDK . We know the bridge bends sideways in the sun, but how far?
