Page 309 hydrography 394 



to be recommended as an anchorage should be sounded thoroughly, the soundings in 

 other parts being frequent enough to give a general idea of the depths. 



The methods described under this heading should be used only as a last resort and 

 when lack of time prevents the use of more accurate methods. 



394. Reconnaissance Survey 



A reconnaissance survey is a hasty preliminary survey of a region made to provide 

 some advance information regarding the area which may be useful pending the execution 

 of more complete surveys. Such a survey is made in a rapid manner, frequently covers 

 an extensive area on a comparatively small scale, and is almost invariably uncontrolled 

 by triangulation. The resulting sui*vey is frequently no more than a sketch of the area, 

 and if soundings are made these are sparse and give only the most general idea of the 

 depths of water in the area. 



The resulting map or chart from such a survey is merely a sketch map and, if 

 published, the representation should not be shown with exactness in a manner to mis- 

 lead the user. The legend of such a map or chart should indicate clearly its quality. 



3941. Running Survey 



Practically all surveys executed by the Coast and Geodetic Survey are parts of an 

 accurately controlled systematic survey of an area. However, in an unknown region 

 far from established control, a running or exploratory survey may be resorted to which 

 will serve at least to determine the general form of the coast and the character of the 

 area, and serve as a reconnaissance for the extension of control into the area and the 

 thorough survey to follow. The results are far from accurate but they do give a 

 rough and superficial examination of an extensive area when time does not permit a 

 more detailed survey. The use of a large scale is unwarranted — a scale of 1:100,000 

 or smaller will usually suffice. 



As the accuracy of the survey depends almost exclusively on the dead reckoning, 

 the errors of the logs and compasses must be accurately determined in advance and 

 the corrections must be carefully applied when the results are plotted. If a taut-wire 

 apparatus is available it may be used to obtain an accuracy in distance impracticable 

 by the use of logs. The accuracy of the courses may sometimes be improved by running 

 the successive courses on ranges selected from time to time, as for example, the tangent 

 of an islet in range with a distant peak. 



In a running survej^ the ship steams along at a safe distance from the coast, its 

 position being fixed by dead reckoning and astronomic observations. Sounding's are 

 taken at the same time. As the ship proceeds along its course, angles and bearings are 

 measured to all identifiable points on the coastline and to conspicuous objects inland, 

 the clock times of the various observations being recorded to correlate the data with the 

 dead reckoning. All off-lying islands, islets, and rocks should be located and their 

 shapes comparatively well delineated. All prominent points along the coast should 

 likewise be located and, where practicable, occasional locations should be made in the 

 bays and indentations between the points, although often the detail between the promi- 

 nent points will have to be sketched. 



The best results are obtained by the use of a moderate and constant speed with as 

 few alterations in course as practicable and without stopping the vessel to make any of 

 the observations. 



