213 HYDROGRAPHIC MANUAL PaGE 62 



For offshore surveys controlled by R.A.R., the required spacing of the stations 

 depends on (a) the size and scale of the survey, (6) the efficiency of reception of sub- 

 aqueous sound, and (c) whether shore stations or sono-radio buoys are used. Sono- 

 radio buoys are usually spaced 10 to 20 miles apart, while shore stations are spaced 15 

 to 30 miles. 



213. Accuracy of Control 



The accuracy of the control stations of a hydrographic survey depends almost 

 directly on the method used in locating the control. The accuracy requirements for 

 control located by triangulation or traverse are given in 223, and the requirements for 

 control located by planetable or air photographic surveys are given in 232. 



When nonstandard methods of locating control are used in places where standard 

 methods are impracticable, they are expected to result in a lesser accuracy than the 

 standard methods for which they are substituted, but special care taken in making the 

 observations and special attention to details should in each case result in an accuracy 

 no less than that of the next lower order of control. 



Definite objects along the shore located by the topographer for the hydrographic 

 party Should be located with the accuracy required in 232. A lesser accuracy may be 

 tolerated in the positions of indefinite objects such as hills and mountain peaks, because 

 they will be used by the hydrographer only when he is at a considerable distance from 

 them. 



Survey buoys, which are used for control beyond the range of shore stations, should 

 be located by the most practicable and accurate method for which the vessel is equipped. 

 Taut-wire sun-azimuth traverses are considered to give an accuracy of approximately 

 1 meter per mile. 



Whether offshore hydrography is controlled by visual three-pomt fixes on shore 

 stations, mountain peaks, or a system of survey buoys connected to shore control, 

 the accuracy of location of any sounding is almost inversely proportional to its distance 

 from the shore. 



214. Recoverable Stations 



Stations artificially marked by recognized surveying organizations and well-defined 

 natural or artificial objects, whose geographic positions have been accurately determined 

 by triangulation, traverse, topography, sextant angles, or other accurate means, and 

 which have been described, are termed recoverable stations. 



Control stations located approximately a century ago were marked by various 

 means such as: redwood stakes on the West Coast; cypress poles, center marked by a 

 copper tack, in marshy areas of the Southern States; earthenware crocks and flowerpots 

 buried in the ground in the Middle Atlantic States; and buried bottles in arid areas. 

 In rocky areas they were marked by. chiseled crosses, plain drill holes, and drill holes 

 into which molten lead was poured or brass bolts or plugs were set. Numerous ingenious 

 types of marks were used which were considered the best practicable with the materials 

 and equipment available. Many of these stations have been recovered in the past and 

 some of them quite recently. Wlien recovered now they are always re-marked accord- 

 ing to present standard specifications (see 2261). 



As the value of more permanent marks became apparent, the trend changed toward 

 marks of a more durable character. Metal marks began to be used in areas where there 

 were no rocks. Among those used were cones, capped rods, and trough-shaped marks 

 for sand beaches and similar areas. Since most of these marks were made of cast iron, 

 it became obvious that, because of rust and corrosion, the problem of permanent marks 



