How is the wavelength of waves measured in the ripple tank practical?

In the ripple tank practical, to measure the wavelength of water waves, first, record the waves on a phone, so the recording can be played back at different speeds and paused. Now measure the wavelength by placing a ruler on the paper. Pause the image of the waves, and measure the distance between the first and tenth wavelength. Finding the wavelength of one wave is then calculated by dividing this distance by 10.