216 



HYDROGRAPHIC MANUAL 



volume, and in the descriptive report or 

 addendum if the rejection impairs the sur- 

 vey coverage. 



When four stations are mistakenly observ- 

 ed and there is not a common center station 

 for 3-arm protracting, the position is fixed 

 by the intersection of two arcs formed by 

 plotting loci of the two angles. 



When the position is weak or indetermi- 

 nate because loci of the two angles approach 

 tangency or actually coincide, acceptance 

 .of the position should depend on criteria 

 (Stated for a single angle position. The plot- 

 ter should be on guard against the "swinger" ; 

 and if this condition is suspected then the 

 position should be protracted a second and 

 third time after moving the protractor com- 

 pletely away from the initial position. A 

 swinger will result in the plotting of several 

 points on an arc. 



Weak positions occur sometimes on elec- 

 tronically controlled surveys where the con- 

 trol is inadvertently used beyond the limits 

 of strong intersections. Conflicting hydrog- 

 raphy may occur at the extended along-shore 

 or inshore limit of the survey where it makes 

 junction with visually controlled hydrog- 

 raphy. Where both systems are used on the 

 same smooth sheet, the visual work in the 

 junctional area shall be plotted first and 

 shall usually be retained where conflict arises 

 from the weaker electronic control. 



Estimated positions are sometimes re- 

 corded when it is not possible, feasible, or 

 necessary to obtain a 3-point fix. They occur 

 close inshore where a normal fix may not be 

 possible, and the distance from the beach, a 

 structure, or station is estimated. Ends and 

 sometimes beginnings of lines are plotted by 

 time interval on the extended line, provided 

 there was no change in speed since the last 

 two positions or before the first two, respec- 

 tively. Structures and objects such as bea- 

 cons, buoys, rocks, piers, etc., are noted in the 

 sounding volume at the moment of passing 

 by the survey launch. These notes afford a 

 check on the survey work, positively record 

 which side of the object was sounded, and 

 also document the existence of features at 

 a specific time. These objects are estimated 



as to observed distance and are recorded 

 at observed time between recorded positions. 

 Skillful evaluation of both elements is re- 

 quired in resolving occasional discrepancies 

 resulting from these estimates. Sometimes 

 when locating a rock or reef it is necessary 

 for the launch to stand off because of rough 

 water, and an estimated distance is recorded. 

 In event of differences with other positions 

 of the same features, considerations should 

 include the possibility of incorrect estimates 

 or unrecorded "stand-offs." 



Estimated positions are necessary to fix 

 the positions of soundings after a survey 

 ship makes a sharp turn and continues to 

 sound. The forward momentum of the ship 

 varies with size, hull and speed, and it should 

 be usually recorded by the boat sheet plot- 

 ting and information recorded by the 

 hydrographer. 



There are places in hydrographic surveys 

 where it is impossible to obtain a three-point 

 fix or other position data especially in nar- 

 row winding sloughs. The hydrographer, in 

 such cases, spots his position on the boat 

 sheet from the adjacent features of the shore 

 line and the note "see boat sheet" is entered 

 in the sounding record. The smooth plotter 

 should transfer such positions, and lines be- 

 tween positions, to the smooth sheet. 



6-50 Erroneous positions. — Two impor- 

 tant qualifications of a competent smooth 

 plotter are his ability to recognize an er- 

 roneous position immediately and the sound- 

 ness of his judgment in deciding the most 

 probable correction which must be made. 

 When parallel straight lines .are run with a 

 uniform interval between positions, an error 

 in position data is immediately apparent 

 when the distance between positions changes 

 without a change of speed otherwise indicated, 

 or when the position plots off the line without 

 a change of course shown in the record. 

 Other errors in positioning may not be dis- 

 closed until discrepancies in depths are 

 found when soundings are plotted. Detached 

 positions not substantiated by check angles 

 should be carefully evaluated when they are 

 plotted. 



Errors usually occur because of the nature 



