This lecture covers the importance of measuring structures for safety and research purposes, focusing on the case study of Riddes bridges over the Rhône. It explains various methods like strain measurement, inclinometry, and direct deformation measurement using fiber optic sensors and strain gauges. The instructor emphasizes the need for systematic measurements, multiple methods, and interpretation of results over time, especially considering the influence of temperature. The lecture also delves into the analysis of inclinometer measurements and solving systems of linear equations to determine structural behavior. Practical aspects such as instrumentation, data documentation, and fieldwork logistics are discussed, preparing students for hands-on experience in structural measurements.