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CHAPTER 2. CONTROL AND SIGNAL BUILDING 



21. CONTROL 



Control is the framework of a survey by which land and marine features are held 

 in their true relationship to each other, and so determined by latitude and longitude 

 that any area surveyed can be reproduced in its correct geographic position on any 

 map or chart, regardless of scale or size. 



At the start of any survey, available data of a higher order are the control for that 

 survey. For example, first-order triangulation is control for second-order triangula- 

 tion; second-order triangulation is control for third-order triangulation; and third- 

 order triangulation is control for topographic, air photographic, and graphic control 

 surveys. The control stations used in the hydrographic survey are ordinarily located 

 as part of one of the last three surveys mentioned, but of course any or all of the con- 

 trol may be so used. 



The control for a survey is a number of accurately located points marked on the 

 earth's surface, strategically situated and spaced with respect to the survey. These 

 points must be fLxed geograpliically in order that they may be plotted on a projection 

 or grid. The principal control points are most accurately determined by triangulation. 

 There are areas, however, where triangulation is difficult and it is more practicable to 

 locate control points by traverse; for example, along a stretch of sand beach, bordered 

 by an adjacent precipitous mountain area. 



Previously established control is a very important asset in a coastal survey. 

 Sometimes a sufficient number of former stations will be recovered to control a revision 

 survey and no new control will be necessary. Most surveys, however, require new 

 triangulation of second- or third-order accuracy, supplemented by topographic and 

 hydrographic stations. 



If triangulation control does not exist, a base must be measured for length control 

 and astronomic observations must be made to determine the latitude and longitude 

 of a station and an azimuth. The astronomic observations should be made preferably 

 at one of the base stations or at one of the stations of the base net. 



Topographic control based on existing or new triangulation is necessary, of course, 

 after the positions of the triangulation control stations are known and available. 



211. Definitions of Control Terms 



Control terms as they apply to hydrographic surveys are defined in order that 

 their precise application may be more thoroughly understood and to prevent ambiguity 

 in their use. 



In general, the correct terminology is dependent on the method of location and 

 type bf instrumental equipment used, modified in certain cases by the accuracy attained 

 and whether or not an adequate check on the location has been provided. 



A triangulation station must be of third-order accuracy or higher, and any sta- 

 tions, located by geodetic methods, which do not comply with the requirements nor 

 attain third-order accuracy, are classified as topographic stations. Among the latter 

 are: (a) Intersection stations located by single triangles, one angle of which is con- 



