273 HYDROGRAPHIC MANUAL PaGE 158 



(A) LIST OF MATERIALS FOR ORDINARY TRIPOD SIGNAL 



Description Size Number required 



Centerpole, S4S 3" by 3" by 16' 1 piece. 



Target, of signal cloth 1" by 4" by 3' 4 pieces. 



or of slats H" by 6" by 3' : or 8 pieces. 



Legs 2" by 4" by 16' 3 pieces. 



Braces for centerpole 1" by 4" by 16' 1 piece. 



For stakes 2" by 4" by 16' 2 pieces. 



Dressing for tripod: 



for whitewashing 1" by 6" by 16' 15 pieces. 



or for signal cloth 1" by 6" by 16') 



o// u A,/u iiiti 4or5pieces. 



or 2" by 4" by 16'j 



Bolt, with nut and washers J4" by 12" 1 each. 



Guy wire (4 guys) No. 8 or 10 4 lengths of 45 feet. 



., , , 14 lengths of 45 feet, 



or 8 guys if needed or< ^ , ., ,,„ , . 



. 1.4 lengths of 30 feet. 



Tools and equipment _ Signal cloth or whitewash, nails, taclfs, pliers, a.xes, saws, hammers, brace and bit, 



plumb bob, shovels, etc. 



273. Steel Towers 



Portable steel towers, identical with those designed for and used in first-order 

 triangulation, have been used for tall hydrographic signals where they were needed at 

 the same time for establishing triangulation control. For details of design and con- 

 struction, see Special Publication No. 158, Bilby Steel Tower for Triangulation. 



The towers are designed in 14-foot sections. This permits them to be dismantled 

 and re-erected at other sites. The total height desired is gained by the addition or 

 omission of the lower sections. 



Transportation is an important factor in getting the steel for the towers to the 

 station sites. A shore building party operating independently from the ship is most 

 satisfactory. The steel can be landed from the ship in small boats if there is no other 

 means of transportation, but this is slow work and pieces may be lost overboard. 



For use as hydrographic signals, targets are necessary, either of canvas or boards 

 bolted to the frame of the tower facing seaward. Since any type of target exposes a 

 comparatively extensiTe surface to the wind, the tower should be guyed accordingly. 

 Since the development and extensive use of buoy control for offshore hydrographic 

 surveys, it is doubtful whether steel towers will be used by the Coast and Geodetic 

 Survey for this purpose in the future. 



274. Water Signals 



Hydrographic signals must occasionally be erected in shallow water several miles 

 from land. Numerous types of signals and methods have been successfully used. 

 Some are partly constructed ashore, towed to the desired location, weighted down, 

 and the construction completed. Others are entirely buUt m the water and still others 

 are built on board vessels and lowered into place. 



The United ^tates Navy has constructed steel towers up to 100 feet in height in 

 depths to 10 fathoms. The material used is similar to that of the Bilby steel tower but 

 the structure is four- instead of three-sided. The three lower sections of the tower are 

 constructed on the deck of the survey vessel and lowered over the side by the boat boom. 

 With the tower suspended in the water from the side of the vessel, other sections are 

 added until the tower rests on the bottom. The vessel is then backed away, the 

 remainder of the construction being completed from motor sailers. The base of the 

 tower is weighted with wooden mattresses filled with sci-ap iron. It can be guyed to 



