The approximate boundaries and the main body of each major current 
shown are based on ship drift observations and direct measurements by 
instrument, which describe the two main features of the current, namely 
direction and speed. Dynamic topography presentations’ are purposely 
omitted but mentioned only where they might prove of interest or value 
by comparison with results of direct measurements. 
The ship drift observations on which many of the descriptions are 
based number in the hundreds of thousands and are retained at the 
National Oceanographic Data Center and U. S. Naval Oceanographic Office. 
These data are basic and most valuable for determining the surface Sues 
systems of large areas and major currents of the oceans. 
The reliability of ship drift observations has frequently been 
questioned. Although external influences, such as wind stress on 
ship superstructure, changing sea conditions, draft of vessel, etc., are 
included in each observation, the effect of these factors is reduced 
in processing the data because only those observations which fall within 
certain coding requirements are recorded, and the remaining factors tend 
to average out as the number of observations increases within a given 
area. For most areas with sufficient observations, a satisfactory 
pattern of surface circulation can be derived, and when compared with 
other methods of observations, for example drift bottles or even short- 
period current meter readings, ship drift is usually preferred. 
The number of current meter measurements at sea is negligible, and 
readings considered reliable or reasonable are shown where available. 
Tt is generally agreed that ocean current information is very 
sparse and usually insufficient for most ocean areas. However, as 
more and better data become available, the presentations and descrip- 
tions in this report can be refined to a greater degree of precision. 
