HYDROGRAPHIC MANUAL 



wherever practicable (see 4—18). The sta- 

 tions shall be established in accordance with 

 Photogrammetry Instruction 45, Revision 1, 

 dated 15 March 1954. Discrepancies in the 

 location of photo-hydro stations should not 

 exceed 0.5 mm. at the scale of the manu- 

 script. 



The Photogrammetry Instructions referred 

 to in this section are part of a series which 

 cover field operations in photogrammetric 

 surveys. A new Topographic Manual, Part 1, 

 will eventually replace them. 



1-17 Planetable surveys. — Instructions 

 for planetable surveys are contained in the 

 Topographic Manual, Special Publication No. 

 144. Topographic surveys by planetable 

 methods will seldom be required ; however, the 

 planetable shall be used to locate signals for 

 control of hydrography wherever expedient 

 to supplement the photogrammetric work 

 (see 4-9). When stations are located by 

 planetable, 90 percent of them shall be with- 

 in 0.5 mm. of their true geographic positions 

 and no station shall be in error by more 

 than 0;8 mm. as measured on the topographic 

 sheet. 



All graphic control and other planetable 

 surveys shall be made on 24 by 31-inch alu- 

 minum-mounted sheets as furnished by the 

 Washington Office. The scale of the survey 

 shall never be smaller than the largest scale 

 hydrographic survey of the same area. 



1-18 Signal location by sextant angles. — 



On occasion it is necessary to locate a signal 

 by sextant angles (see 4-26). In such cases 

 the position of the signal shall be determined 

 by (1) a three-point fix at the station with 

 a check angle; or (2) three sextant cuts 

 giving a good intersection. The geographic 

 position of the station should not be in error 

 by more than 1.0 mm. The strongest avail- 

 able fix should always be used and should 

 include triangulation stations where prac- 

 ticable. Stations located by sextant angles 

 shall not be used to locate other stations. 



1-19 Electronic control. — The shore sta- 

 tions for electronic control of hydrographic 

 surveys shall be established at triangulation 

 stations, or the principal antenna shall be 



located by triangulation of at least third 

 order accuracy. Distance arcs at fixed posi- 

 tions should not intersect at angles less than 

 30° or more than 150°. Three electronic 

 systems are authorized for use in position 

 fixing: Shoran, Electronic Position Indicator 

 (EPI),andRaydist. 



(a) Shoran is a line-of-sight system and 

 should rarely be used at greater distances 

 (see 3-33 to 47). The system requires very 

 careful and repeated calibration. At least 

 one calibration for each ground station shall 

 be obtained for each 5 consecutive days. Cali- 

 brations shall be obtained at various dis- 

 tances including the minimum and maximun 

 distances used in hydrography. Whenever 

 changes are made in the equipment, the time 

 of change must be recorded and new cali- 

 brations observed. A zero check shall be 

 observed and recorded at each calibration, at 

 the beginning and ending of each day's work, 

 and at intervals of 1 to 2 hours while sur- 

 veys are in progress. Special manuals are 

 provided containing instructions for instal- 

 lation and servicing of Shoran equipment. 



(b) EPI equipment is designed for use on 

 large survey ships to control offshore hydrog- 

 raphy at maximum ranges of about 500 nau- 

 tical miles (see 3-23 to 32). It shall not be 

 used to control surveys at a scale larger than 

 1:100,000. The equipment shall be calibrated 

 at the beginning and end of each trip, when 

 changes are made in the equipment, after 

 each period when the equipment is not in 

 operation, and at other convenient times. De- 

 tailed instructions for the operation and 

 maintenance of EPI equipment are con- 

 tained in Special Publication No. 265A, 

 EPI Manual. 



(c) The standard Raydist equipment is 

 designed for use in survey vessels of any 

 size larger than launches (see 3^8 to 61). 

 A smaller transistorized equipment can be 

 used in small survey boats. Raydist may be 

 used to control surveys at any scale nor- 

 mally used by the Bureau. The phasemeter 

 dials must be set at a known position. Par- 

 tial lane corrections to measured distances 

 shall be applied when they can be plotted at 

 the scale of the survey. The lane count must 



