Page 233 hydrography 3371 



From its very nature, dead reckoning is far from exact, its accuracy depending on 

 the estimate of the run which is always apt to be more or less in error. A position 

 determined by dead reckoning is likely to be in error by an amount proportional to the 

 distance from the last known position. Errors which are scarcely subject to measure- 

 ment may be introduced in a number of ways; among those affecting the course are 

 imperfect steering, incorrect allowance for compass error, leeway, and current; and 

 among those afTecting the distance are imperfect logs or unknown log factors, and 

 incorrect allowance for current. 



3371. Prevalent Uses of Dead Reckoning 



Besides its use to supplement fixed control, the most extensive uses of dead reckon- 

 ing in hydrographic surveying are to control or assist in controlling the position of 

 the survey ship beyond the range of control stations and to supplement astronomic 

 observations. Hydrography beyond the range of control stations should not be 

 controlled by dead reckoning for more than 8 consecutive hours, weather permitting 

 astronomic observations; lines taking more than 8 hours should be planned to be 

 fixed by astronomic sights. The three commonest cases in which dead reckoning 

 is used are: 



(1) Where the control stations cannot be used to the limits of the area to be 

 surveyed. In this case the dead-reckoning lines are run offshore, usually normal to the 

 trend of the coast, to the limit of the area. The dead-reckoning line is almost always 

 an extension of a well-controlled sounding line, fixed by the usual methods as far 

 offshore as possible. The fixed positions farthest seaward aid in determining the 

 correct course to be steered offshore and give a value for the leeway and current at 

 the beginning of the loop. As the ship proceeds seaward the course is changed to allow 

 for estimated variations in leeway and drift, thus maintaining the course to be made 

 good, insofar as it can be estimated. Wlien the ship arrives at the offshore limit of the 

 area being surveyed, the course is changed about 90° for the run between adjacent 

 sounding lines; it is then again changed about 90° to a course opposite to the original 

 one until a fixed position can be obtained by conventional methods. This last course 

 should be established by taking into consideration the estimated leeway and drift and 

 continued without change, except for allowance for variation in leeway and drift, until 

 a fixed position has been obtained. The line should be continued for several positions 

 after the first fixed position, to establish the course made good and the log factor at this 

 end of the line. Account should be taken of the turning radius of the ship at the two 

 turns at the offshore end of the line (see 3373) . 



Between the fixed positions farthest offshore other partial data should be obtained 

 whenever possible to aid in the adjustment of the line (see 3378). Such a dead-reckon- 

 ing loop is rarely long enough for astronomic observations to be of much value, but 

 loops of more than 8 hours' duration should be planned so that the outer end may be 

 fixed astronomically. In the latter case clear weather should be selected and the line 

 should be started at a time for the outer end to be reached at dawn or dusk, when a 

 good set of stellar observations may be obtained. Such observations should be taken 

 on the short course between the two sounding lines. 



(2) Short loops similar to the above may be run radiating from a buoy, usually 

 centrally located on a shoal. In this case the lines are comparatively short, and 

 astronomic observations are of no value. Such lines usually start from a position close 

 to and abeam of the buoy, the nearer positions being fixed by bearings and vertical 



