A basic sampling problem that arises when the unambiguous velocity sampling interval is less than the full range of naturally occurring velocities, causing the erroneous appearance of higher velocities within the sampling interval. This phenomenon occurs in Doppler velocity measurements when the maximum unambiguous velocity interval (±V_max) is less than the full range of velocities being measured. Any true velocity, V, appears within the interval from −V_max to +V_max, with the value V′, which is related to the true velocity by V = V′ ± 2nV_max where n is an integer. Therefore a given measured velocity V′ may be caused by many values of the true velocity V. For example, suppose V_max = 25 m s⁻¹ and the measured velocity V′ = −15 m s⁻¹. Then the values of true velocity that could account for this measurement are the following: −15 m s⁻¹ (for n = 0); +35 or −65 m s⁻¹ (for n = 1); +85 or −115 m s⁻¹ (for n = 2); etc. In some instances the erroneous velocities can be recognized and ambiguities resolved by additional considerations, such as the requirement of spatial continuity of the velocity field. See Also aliasing, Nyquist frequency.