It has, of course, always been recognized that one of the greatest uncertainties of dynamic oceanographic calculations is that which results from the necessity of selecting without definite evidence a surface of assumed zero motion to which the calculated relative velocities can be referred for evaluation in absolute terms....
Parr, A. E.. 1938. "Analysis of current profiles by a study of pycnomeric distortion and identifying properties." Journal of Marine Research 1, (4). https://elischolar.library.yale.edu/journal_of_marine_research/533