NGC4321 is a two-armed grand design spiral galaxy in the Virgo Cluster. HI emission line data with 15 arcsecond resolution from the VLA map the velocity field of the hydrogen gas in the disk of NGC4321. In a grand design spiral galaxy, the presence of a spiral density wave causes deviations from circular rotation in the gas. The observed velocity deviations owing to the spiral density wave can be resolved into a part that varies sinusoidally with azimuth and a part that varies trisinusoidally with azimuth. The strength of the trisinusoidal part grows with radius relative to the strength of the sinusoidal part. Knowing the amplitude of the velocity deviations due to the spiral density wave provides a measurement of the pattern speed of that wave. Circular motion in an inclined disk also has a sinusoidally varying component. Partitioning the total sinusoidal component into the part due to projected circular motion and the part due to the spiral density wave is thus ambiguous without extra information. The ambiguity can be resolved by considering the growth with radius of the trisinusoidal component in the velocity perturbations due to the spiral density wave. We use this approach to analyze the observed HI velocity field of NGC4321 and thereby to discover the resonance structure of the two-armed spiral pattern.