to detect by means of a long - baseline system the global bending strain of a structure , i . e . by detecting the accumulated local bending strains over a distance of at least two meters , detection lines , preferably optical fiber lines , which indicate the extent of their elongation by an increased attenuation of the light pulses which run through them , are used . to detect the direction and the extent of the flexure of the structure , the latter is closely associated with a model comprising a matrix which follows the flexural bending strains of the structure , and at least one , preferably two detection lines suitably located in the matrix cross - section so that their elongation variations and / or the differences between their respective elongation variations are indicative of the flexure of the model , which gives indications on the flexure of the structure . the invention is useful in determining the straight or distorted flexure or contraflexure of structures not easily accessible or partially inaccessible , to measure level variations on one or several axes .