3941 HYDROGRAPHIC MANUAL PaGE 310 



A minimum of three or more directions should be observed to each object as a 

 check both on the accuracy of the work and the identification of the object. It must 

 be expected that these directions will usually not intersect at a point, but the triangle of 

 error shoud be small enough to ensure that the observations have been taken to the 

 same point and that there will be little difficulty in selecting an acceptable position. 

 Rapid sketches, made at the different positions, of the details to which the observations 

 were made will be of material assistance in correcting and adding to the data when 

 plotted. 



If the depths permit, a still more accurate and complete running survey may be 

 based on a series of buoys anchored by the ship during its dead-reckoning line, the 

 distances and azimuths between them being measured by taut wire and sun azimuths 

 from the ship. After this line of buoys is m place and their positions are available, a 

 hydrographic survey of the area and a survey of the shoreline to any extent may be 

 carried out. 



Each nmning survey should start and end with a fixed position. Such positions 

 can rarely be related to the basic control; generally they are based on astronomic obser- 

 vations. Wlien the astronomic observations at the ends of the line and during the dead 

 reckoning have been computed and positions have been determined, all of the data 

 should be replotted and readjusted to these positions. 



The amount of detail to be obtained during a running survey will depend on the 

 amount of time available. Additional detail, especially of the inshore area, may be 

 obtained by employing a launch in conjunction with the ship to sketch in the smaller 

 detail, the position of the launch being located from time to time with reference to the 

 ship at a prearranged signal between the two. The launch itself may run a dead-reckon- 

 ing course which is related to the ship's dead reckoning, takmg soundings at the same 

 time. Better results are probably obtained by dispensing with the sounding and letting 

 the launch proceed at will to various strategic points where it stops to measure angles 

 and sketch in detail, each position of the launch being referred to the ship's dead 

 reckoning. 



All of the observations should be recorded and all bearmgs and angles should be 

 plotted at the time they are observed in order to detect any errors in identification or 

 measurement while the detail to which the observations were made is still in view. 



In making a running survey a number of observers and recorders are essential and 

 the various phases of the work must be divided between them so that each has a definite 

 duty to perform and not more than he can do efficiently. One observer usually 

 takes astronomic observations and supervises the dead reckonmg; two or more may be 

 employed in identifying shore objects and measuring angles and bearings to them; 

 another may be employed in identifying various hills and summits and measuring hori- 

 zontal and vertical angles to them. One person experienced in that kind of work 

 should make sketches of the coast which may be related later to the points which have 

 been located. 



Regardless of the method used in a running survey or the accuracy attained there 

 will be discrepancies in the results and the hydrographer has the problem of effecting a 

 satisfactory compromise, which can best be done only by one who has had long experi- 

 ence in this type of survey. 



