topography, these results may indicate that the periodic currents 

 of periods of more than a few hours were small enough to have 

 been masked by larger aperiodic fluctuations, or that the area con- 

 sidered was large enough to include geographical differences in 

 the periodic currents. 



Since direction of the GEK vector is better than its magnitude, 

 some means of constructing a current chart was sought in which 

 'the complete vector was used instead of using only one component 

 of the vector. 



The method of producing a current chart from current vectors 

 gotten from the GEK is essentially the determination of the stream 

 function (i|0 for many points on the chart. Contouring the chart 

 for lines of equal value of the stream function will result in a cur- 

 rent pattern, since the stream function is defined by this premise. 

 The validity of the results depends on three major assumptions: 

 that the GEK vectors are truly representative of the current, that 

 the currents over the survey area are horizontal and free from 

 vertical motion, and that the points at which a GEK current fix 

 is obtained are spaced closely enough to permit representative 

 sampling of all important spatial current fluctuations. 



The first assumption is dependent upon the instrument and is 

 reasonably safe in that sufficient observations have shown a good 

 correspondence between GEK vectors and true currents. 



The second assumption of horizontal flow is also an assumption 

 in the computation of currents by the method of dynamic topog- 

 raphy. However, there is a difference in that in computing the 

 stream function any errors introduced due to divergence will be 

 cumulative from station to station and may become very important 

 after a number of stations if not kept under control. Maintaining 

 control of the divergence is an important phase which is reflected 

 both in laying out the track of the survey and the method of com- 

 puting the stream function. 



The third assumption of adequate sampling merely requires that 

 GEK stations be made frequently along a track line such that all 

 major current variations are in evidence. 



The stream function (xj^) of fluid flow at a point x, y is defined 

 mathematically as: 



yjy,, y= I {u dy-v dx) 



Xq, 7/0 



where Xq, iJq is an arbitrary starting point where ^\)=io; u and v 

 are the x and y components respectively of the velocity. 



If the GEK stations are spaced closely enough along a track line 



36 



