String pots/ Draw Wire Sensor
String pots are a wonderfully simple concept for measuring linear position. The cable winds onto a spool which is sprung loaded converting linear measurement into rotation. This rotation is measured using a potentiometer or encoder which is in turn converted to an analogue or digital output. They are easy to install, no critical alignment is needed and makes a direct measurement at the point of interest.
A wide variety of options in regards to cable actuated, string pots, draw wire type sensors to cover many applications for lengths beyond 50 metres. Each data sheet has many options to cover your application for structual testing, automotive testing, civil/structural or even OEM requirements such as boom arm, elevators and outriggers.