A method for improving determination of mode shapes for a mechanical structure where each mode shape is a vector that consists of a number of components and each vector corresponds to a natural frequency of the structure where the method is based on • Measurements performed on the structure using signals from a limited number of sensors such as accelerometers placed on the structure defining the number of components of each of the mode shape vectors, where the measured signals are used to determine a number of mode shapes a 1, a 2 ···, which are listed according to natural frequency starting with the lowest natural frequency. • Determination of a number of mode shapes b 1, b 2..., based on a numerical model of the structure, defining a number of components of each mode shape b in the numerical model, each of which corresponding to a mode shape a calculated from the measurements, wherein the number of components of each mode shape b in the numerical model is larger than the number of sensors. The mode shapes b 1, b 2 ··· are listed according to natural frequency starting with the lowest natural frequency. • Determination of a set of modified mode shapes â 1,â 2 ··· for improving the accuracy of the mode shapes a 1,a 2... by removing influence from random noise and measurement errors, where each of the modified mode shapes â is an improvement of the mode shape a corresponding to the mode shape b in the numerical model, where each of the modified mode shapes â is defined as the best fit of a linear combination of the mode shapes b 1, b 2... but reduced to the set of components corresponding to components in the measured signals, where a value expressing the difference between the mode shapes a calculated from the measurements and the corresponding modified mode shapes â is minimized. Characterized in that the linear combination consists of a limited number of the mode shapes b 1, b 2... with natural frequencies around the natural frequency of the corresponding mode shape b. • Measurements performed on the structure using signals from a limited number of sensors such as accelerometers placed on the structure defining the number of components of each of the mode shape vectors, where the measured signals are used to determine a number of mode shapes a 1, a 2 ···, which are listed according to natural frequency starting with the lowest natural frequency. • Determination of a number of mode shapes b 1, b 2..., based on a numerical model of the structure, defining a number of components of each mode shape b in the numerical model, each of which corresponding to a mode shape a calculated from the measurements, wherein the number of components of each mode shape b in the numerical model is larger than the number of sensors. The mode shapes b 1, b 2 ··· are listed according to natural frequency starting with the lowest natural frequency. • Determination of a set of modified mode shapes â 1,â 2 ··· for improving the accuracy of the mode shapes a 1,a 2... by removing influence from random noise and measurement errors, where each of the modified mode shapes â is an improvement of the mode shape a corresponding to the mode shape b in the numerical model, where each of the modified mode shapes â is defined as the best fit of a linear combination of the mode shapes b 1, b 2... but reduced to the set of components corresponding to components in the measured signals, where a value expressing the difference between the mode shapes a calculated from the measurements and the corresponding modified mode shapes â is minimized.