Before answering this question, first we have to understand a transverse wave.
A transverse wave is a type of mechanical wave in which the vibration of particles is perpendicular to the direction of wave propagation. In this type of wave one will find crests and troughs. A transverse wave can be considered as a series of crests and troughs.
The characteristics of a transverse wave are wavelength, frequency and amplitude.
The wavelength of a transverse wave is the distance between two successive crests and troughs. Hence, the wavelength is measured by simply calculating the distance between either crests or troughs. It may be also calculated between two successive points which are at same phase.
The frequency of a transverse wave is defined as the number of waves passing through a point per second. One wave corresponds to one wavelength. Hence, the number of wavelengths of a wave per second is known as the frequency of the wave.
The amplitude of wave is calculated as follows. The amplitude of a wave is defined as the distance of the mid point of any half cycle of the wave to its crest or trough. On the other hand, it can be defined as the maximum displacement of vibration of the particle from its mean position.