than 10 miles. The position of each ship was determined at 15-minute 



time intervals and then plotted on Mercator Plotting Sheets (scale: 



o 

 1 longitude = 4 inches). Near land areas, Loran-C or Decca was 



usually available. Here the maximum position error is less than 1 mile. 



The relative positions of the ships were maintained by taking occasional 



radar ranges and bearings from one ship to another. 



After completion of the survey, each ship's smooth-plotted survey 

 track was transferred to a master set of plotting sheets. The best 

 position fixes from each ship were combined with the intership radar 

 range and bearing data. Consequently, the resulting adjusted tracks 

 represent a compilation of the best possible positioning information. 

 C. Instrumentation 



Each survey ship was equipped with a Varian nuclear resonance 

 magnetometer, Model XN-4901. With this instrument, the absolute value 

 of the total magnetic field intensity can be measured to an accuracy 

 of about +1 gamma. The sensing unit was towed 500 feet astern to 

 reduce the effect of the ship's magnetic field. Data were recorded on 

 a Varian G-11 analog recorder in units of "magnetometer counts." 

 These units, an inherent property of the magnetometer design, are an 

 inverse function of the total magnetic field intensity. 



Each ship also was equipped with a standard Edo AN/UQN Sonar set. 

 Instrument output was recorded directly in fathoms on a Mark VI Pre- 

 cision Depth Recorder. 



Greenwich Mean Time was used exclusively for all survey work. 

 The time marks for both the magnetometer and the Precision Depth 

 Recorder were provided automatically by a central clock on each ship. 



