A method of estimating a radial deformation of a rotary shaft in a loaded condition, comprising: recording a first radial displacement of the shaft's surface while rotating the shaft at a first angular speed corresponding to a relaxed condition of the shaft; on the basis of the first radial displacement, providing a first shape description representing the shaft's cross section in the relaxed condition; recording a second radial displacement of the shaft's surface while rotating the shaft at a second angular speed corresponding to said loaded condition of the shaft; tracking a reappearing local irregularity in the recorded first or second radial displacement; on the basis of the second radial displacement, providing a second shape description representing the shaft's cross section in the loaded condition; and determining the radial deformation as a difference of the first and second shape descriptions, characterized in that the method further comprises: (a) identifying the local irregularity with a structure that has a known radial dimension and scaling the recorded first or second radial displacement to match said known radial dimension; or (b) scaling the recorded first or second radial displacement non-uniformly with respect to the recording-time dimension or angle dimension, such that the tracked local irregularity reappears with an equal separation in recording time or angle, to compensate any variations in angular speed between the revolutions. recording a first radial displacement of the shaft's surface while rotating the shaft at a first angular speed corresponding to a relaxed condition of the shaft; on the basis of the first radial displacement, providing a first shape description representing the shaft's cross section in the relaxed condition; recording a second radial displacement of the shaft's surface while rotating the shaft at a second angular speed corresponding to said loaded condition of the shaft; tracking a reappearing local irregularity in the recorded first or second radial displacement; on the basis of the second radial displacement, providing a second shape description representing the shaft's cross section in the loaded condition; and determining the radial deformation as a difference of the first and second shape descriptions, characterized in that the method further comprises: (a) identifying the local irregularity with a structure that has a known radial dimension and scaling the recorded first or second radial displacement to match said known radial dimension; or (b) scaling the recorded first or second radial displacement non-uniformly with respect to the recording-time dimension or angle dimension, such that the tracked local irregularity reappears with an equal separation in recording time or angle, to compensate any variations in angular speed between the revolutions. (a) identifying the local irregularity with a structure that has a known radial dimension and scaling the recorded first or second radial displacement to match said known radial dimension; or (b) scaling the recorded first or second radial displacement non-uniformly with respect to the recording-time dimension or angle dimension, such that the tracked local irregularity reappears with an equal separation in recording time or angle, to compensate any variations in angular speed between the revolutions.