Anyone who knows me knows how much time I spend playing with (I prefer to think of it as developing) ways of measuring things on big structures. I recently had occasion to measure sideways deflections at the top of tall bridge piers. There will be more details later but I have found it possible to measure deflections from up to say 200m using a video camera, a telescope and a target. the video Download moire_target_trials.wmv (349.5K) shows the results of an early trial on a small pier. The first 9 seconds is a plain video the repeat has an overlay to create a fringe pattern to amplify the movement. The video is an iPod grade one so you might need quicktime or an ipod to be able to play it. It is clearly possible to see the circular motion as the passing train pushes the pier to the left, down and round.