i am not William, but i was reading this:
"Velocity vectors obtained with poor correlations are considered invalid and are referred to as outliers. The source for the outliers can be from three-dimensional motion, large velocity gradients, improper particle seeding and so on. Detailed studies can be found in many previous publications (Keane and Adrian
1990, Willert 1992, Huang et al 1993a, Westerweel 1993)"(Huang et al., 1997)
In his study, these conditions are difficult to control. I think that's how it is
I think that one solution could be to take photos from away and very consecutive photos 10/1s (so so).