The present invention defines a method of determining a vertical profile signal of a rail surface that includes, obtaining a vertical acceleration signal acc1, by measuring vertical acceleration of a bogie of a rail vehicle that runs on the rail surface; processing the vertical acceleration signal to obtain a vertical velocity signal; determining the vertical profile signal of the rail surface, by using the vertical acceleration signal and the vertical velocity signal as inputs to a simulation model of the bogie, the model having an unsprung mass connected to a sprung mass, the vertical acceleration signal acc1 represents the vertical acceleration of the unsprung mass; and measuring a linear velocity signal of the rail vehicle, the linear velocity signal is used in the step of determining to convert the vertical profile signal from the time domain to the distance domain.