5. HYDROGRAPHY 



147 



changes of course are required, the frequency 

 and amount of change depending on the 

 radius of the arc being followed. It is pos- 

 sible to follow the curve very closely ; how- 

 ever, there will usually be periods when the 

 vessel is slightly off the desired course. The 

 courses need not be recorded for launch hy- 

 drography but should be recorded for ship 

 hydrography. 



5-45 Position fixing by EPI.— The EPI 

 system (see 3-23 to 32) is generally used 

 to control small scale offshore hydrographic 

 surveys. The position of the survey ship is 

 determined by measuring electronically the 

 distance to two fixed stations ashore. Dis- 

 tances are measured in units of microseconds 

 (one microsecond =149.8447 m). The sys- 

 tem does not provide the accuracy of Shoran 

 or Raydist; however, it will provide satis- 

 factory control for surveys plotted at a scale 

 of 1:100,000 or smaller if the equipment is 

 properly installed, adjusted and operated. 

 The operators at the shore stations must per- 

 form their specified duties with great care 

 or the position data will be inaccurate. The 

 plotted position should always be compared 

 with the dead reckoning position, and, if 

 there is much difference between them an- 

 other fix should be observed and plotted. It 

 is necessary to reject one or both distances 

 occasionally on the basis of a reasonable eval- 

 uation of the dead reckoning and a series 

 of fixes. 



5-46 EPI control procedure. — Chapter 3 

 of the EPI Manual describes in detail the 

 operation of obtaining an EPI fix. The in- 

 terval between fixes depends on the scale of 

 the survey and the speed of the ship and 

 usually varies from 10 to 30 minutes. 



Since the use of EPI is restricted to con- 

 trol of small scale surveys in offshore areas, 

 the boat-smooth sheet method of plotting the 

 survey should be used in most cases (see 

 5-5). The calibration program described in 

 3-29 and in the EPI Manual must be carried 

 out before hydrography is begun and re- 

 peated as necessary. The effect of the ship's 

 heading should be determined (see 3-30) 

 and a table of corrections compiled and 



checked. A new table of corrections may be 

 required as a result of subsequent calibra- 

 tions. Calibrations obtained on different 

 dates should not be meaned. 



When a fix is taken the EPI readings 

 shall be recorded in the sounding record and 

 on the abstract (Form 817). The observed 

 distances are corrected on the abstract in 

 accordance with the correction table and the 

 position plotted. The EPI dials should not 

 be moved until the position has been plotted 

 and accepted. The abstracts and tables of 

 corrections shall be submitted with the other 

 survey records for the sheet. 



5-47 Position fixing by Raydist. — The 



DM Raydist system (see 3^8 to 61) is com- 

 pletely automatic. Once the phasemeter dials 

 have been correctly set at a known position, 

 the movements of the vessel are recorded on 

 the brush recorder and shown on the dials. 

 A fix may be obtained at any instant by 

 reading the lane count from the phasemeter 

 dials. Errors in measured distances may 

 be introduced by radio interference, sky 

 wave contamination, loss of power, or elec- 

 trical storms. It is therefore essential that 

 the brush recorder be under continuous ob- 

 servation to detect any gain or loss of lanes. 

 These changes will occur in whole units and, 

 if conditions are not too severe, the readings 

 can be corrected. If the lane count is lost 

 or is uncertain, it is necessary to set the 

 dials again at a known position or calibra- 

 tion point. 



Raydist may be used to control surveys at 

 any scale; however, when plotting on a 

 scale larger than 1:20,000, there are a few 

 small corrections to be applied as described 

 in 3-56. 



The position of the sounding vessel is de- 

 termined by the intersection of two distance 

 arcs expressed in terms of lanes. The width 

 of a lane is a function of the radio fre- 

 quency used in the system. 



5-48 Raydist control procedure. — A sys- 

 tem of parallel straight sounding lines may 

 be run, or a system of curved lines along 

 selected distance arcs may be run as with 

 Shoran. In either case fixed positions shall 



