146 



HYDROGRAPHIC MANUAL 



the intersection of two arcs centered on the 

 known points with radii equal to the meas- 

 ured distances. Data for plotting a position 

 is read directly from dials to 0.001 statute 

 mile. 



When properly adjusted and operated, 

 Shoran provides very good control for sur- 

 veys at scales of 1 : 20,000 and smaller. For 

 work at larger scales it is necessary to be 

 more careful in calibrating the equipment 

 and to make frequent checks on the accuracy 

 of the data it furnishes. 



5-42 Zero set and zero check. — Zero set 



is an adjustment of a reference pulse marker 

 (see 3^3) and zero check is a procedure for 

 observing the stability of zero set (see 3-44) . 

 The zero set is adjusted during the calibra- 

 tion procedure to provide a minimum cor- 

 rection to observed distances. Any departure 

 from the original setting as shown by pe- 

 riodic zero checks represents an additional 

 correction to be applied to measured dis- 

 tances. Ordinarily these corrections are 

 small and can be ignored for boat sheet 

 plotting. If the range of the zero check 

 corrections does not exceed .005 statute mile, 

 the zero checks can be meaned, or ignored 

 if the correction is not plottable at the scale 

 of the survey. 



If successive checks show a radical change, 

 0.010 statute mile or more, the cause should 

 be determined and corrective measures tak- 

 en. Such a change may be caused by loose 

 antenna connections. 



5-43 Reliability of Shoran. — There are 

 times when Shoran distance measurements 

 are inaccurate although the equipment seems 

 to be functioning properly. The hydrog- 

 rapher must be constantly alert to detect 

 indications of errors in the Shoran. A sum- 

 mary of the precautions to be used is con- 

 tained in 3^7. 



One of the major problems in verification 

 of Shoran controlled surveys is encountered 

 at junctions of adjacent surveys or at junc- 

 tions of surveys by two or more vessels on 

 the same survey sheet. Calibration correc- 

 tions should be applied when plotting posi- 

 tions on the boat sheet in order that junc- 



tions with other surveys can be properly 

 evaluated. Any discrepancies must be re- 

 solved in the field. A three-point fix should 

 be observed occasionally for comparison with 

 a Shoran fix. It is obvious that when the 

 distance arcs intersect at an acute angle, 

 small errors in the distances produce a larger 

 displacement of positions than will occur 

 when the arcs intersect at an angle of about 

 90°. Comparison fixes are desirable in all 

 areas of the survey. 



5-44 Shoran control procedure. — The 



procedure to be followed in matching pulses 

 and reading Shoran distances is described 

 in the Shoran manual and will not be re- 

 peated here. When Shoran is used to con- 

 trol a hydrographic survey, there is a se- 

 quence of events which should be followed for 

 best results. 



The entire system, both ship and ground 

 stations, should be started at least 30 min- 

 utes before operations begin. Check and re- 

 cord the zero setting. Check the ship station 

 oscillator with the ground station and, if 

 necessary, make the adjustment for calibra- 

 tion synchronization (cal sync) . 



If circumstances permit, a series of cali- 

 bration comparisons should be made (see 3- 

 38 to 40) before surveys are begun. A pre- 

 liminary table of corrections versus distance 

 should be compiled for each shore station 

 and posted at the plotting table. A zero 

 check reading shall be recorded at each series 

 of calibrations and once each hour while 

 hydrography is in progress. 



The Shoran readings at each fix shall be 

 recorded in the sounding record. Form 275, 

 and on the Shoran plotting abstract. Form 

 817. The observed distances shall be cor- 

 rected on the abstract in accordance with 

 the posted table before the positions are 

 plotted on the boat sheet. The dials on the 

 Shoran Indicator should not be turned until 

 the position has been plotted and accepted 

 as satisfactory. Occasional errors in read- 

 ing the dials, or errors in recording the data 

 can be corrected at once if the dials have 

 not been moved. 



When sounding lines are being run along 

 preselected Shoran distance arcs, frequent 



