Example of the pattern speed measurement for a single spiral arm in model M1. Left: coloured lines correspond to the sum of the squared-root differences between the stellar density at t₀ = 0.6 Gyr and the ones from t₀ ± 1 Myr rotated by a certain angle ϕ (see Eq. (1)). The minimum value corresponds to the angle of rotation which being divided by 1 Myr results in the pattern speed value (black symbols) at a given R_gal. Right: pattern speed of the spiral arms in different models. Values for the individual spiral arms are shown by the error bars of different colours. For a better representation, the pattern speed for each arm is shifted vertically by 10 km s⁻¹ kpc⁻¹ and compared to the circular frequency (solid lines) which is also shifted by the same values.