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1132. Hydrographic Surveying 



As used in the Coast and Geodetic Survey, hydrographic surveying is one phase of 

 combined operations and in this hmited sense may be said to consist of three essential 

 operations — sounding; location of soundings, and aids and menaces to navigation; and 

 records: 



(a) Depth measurements, usually called soundings, are made throughout the area being surveyed. 

 This includes the determination of the nature of the bottom at selected intervals, and the measure- 

 ments of some of the physical characteristics of sea water, such as water temperatures and salinities 

 at various depths. Data may be collected for oceanographic research, such as specimens of bottom 

 material and water samples. 



(6) The positions of the soundings are determined by reference to control stations; numerous 

 features, such as rocks, reefs, wrecks, and aids to navigation, are located and examined so that they 

 may be charted for the guidance of navigators. 



(c) All data are recorded and plotted to provide permanent records of all the information ob- 

 tained during the survey and to serve as a basis for the construction of nautical charts and the com- 

 pilation of Coast Pilots. 



1133. Survey Projection and Scale 



Although most nautical charts are constructed on the Mercator projection, all 

 original field surveys are plotted on the polyconic projection which is especially useful 

 for this puipose (see 7321). 



The scales of the surveys are chosen with respect to the amoimt of detail in the 

 area and are usually at least twice as large as that of the largest-scale chart to be pub- 

 lished of the area. The scales are almost invariably multiples of the proportion 

 1:10,000. (See 123.) 



1134. By-product Value of Surveys 



In addition to their use in chart compilation, hydrographic surveys and other 

 combined operations have an important by-product value. For many parts of the 

 coast the topographic surveys are on a larger scale than are available elsewhere, and 

 for some areas, especially outside the continental limits of the United States, they are 

 the only topographic surveys existent. The results of the surveys are valuable for 

 military defense, for planning harbor improvements, for the study of physical hydrog- 

 raphy, beach erosion and other shoreline changes, and for oceanographic studies. 



For much of the coastal area the early topographic surveys of the Bureau are the 

 earliest authentic records of shoreline conditions and the detailed topography of the 

 adjacent terrain. These and the periodic revision surveys of the constantly changing 

 shoreline have a historic value, in addition to their value in land disputes. 



The periodic hydrographic and topographic surveys are of great and increasing 

 importance in the study of shoreline changes. The history of beach evolution during 

 the past century can often be deduced from the records of the Bureau, with the result 

 that the engineer can frequently determine the character of the forces which he seeks 

 to control. 



Students of the earth sciences have been handicapped in the past by their 

 inability to investigate the vast unknown areas beneath the ocean surface. Except for 

 the fringe along the coasts, soundings, even where taken, were so widely spaced and so 

 inaccurately controlled in position as to give little basis for scientific conclusions. 

 Gradually, with the perfection of echo sounding and more accurate methods of control, 

 the submarine relief is being revealed, particularly that of the submerged continental 

 margins. It is now possible not only to discover and chart the submarine features 



