Page 153 control and signal building 272 



serve that can be identified from a distance of several miles. Where natural objects are 

 not available, the most satisfactory and economic types of signals are those made of 

 whitewash or white signal cloth. These show very distinctly against a dark back- 

 ground when direct sunlight is reflected from them, but will be visible only a short 

 distance if they are in shadows or during overcast weather. Consequently, it is very 

 important to dress the signals in direct contrast to the background, i. e., white against 

 a dark background and black against the skyline, or a combination of black and white 

 when the signal shows against a light background from one direction and a dark back- 

 ground from another. Red is the most satisfactory color to use against a background 

 of snow, for a black target is easily confused with protruding rocks. 



For marking stations and erecting whatever types of signals may be needed in any 

 particular region, the stores of survey vessels should include ample supplies of unslaked 

 lime, signal cloth, canvas, slats, lumber, cement, cast-iron soil pipes, wire, bolts, nuts, 

 nails, tacks, tools, etc. Muslin signal notices (Form 51) shall also be carried and used 

 wherever there is a likelihood of signals being disturbed or destroyed by persons 

 unaware of their purpose. 



Where stations and signals are to be established on private property in inhabited 

 areas, permission must be obtained from the owner, and no damage to or defacement of 

 property shall be made without his consent. 



Where surveys are made along the shores of publicly owned areas, such as public 

 parks, National or State forests, reservations, and wildlife refuges, the superintendent 

 or other official must be contacted to explain the nature of the survey and to request 

 his cooperation. This is especially important where marks are to be established, signals 

 erected, or observing lines cleared through wooded areas. Satisfactory cooperation can 

 usually be obtained when the nature of the work is understood, and arrangements can 

 often be made to have one of the caretakers accompany the party for a day or two to 

 approve the necessary clearing and help dispose of the resulting wood and brush. He 

 may be hired for this purpose except when he is a federal employee. 



Where signals are erected over marked stations, precautions must be taken not 

 to disturb the station and reference marks. 



For signal buildmg, reference should also be made to Special Publication No. 234, 

 Signal Building. 



272. Tall Signals 



Since the extensive use of buoys for offshore hydrographic control, the need for 

 very tall signals has diminished during the last 10 years. In recent hydrographic 

 surveys along low flat coasts, 40 feet has been about the maximum height needed to 

 furnish ample control for distances of 5 to 10 miles offshore, from which limit the 

 hydrography is extended by buoy control. 



Of the types of tall signals used by the Coast and Geodetic Survey in the past, 

 wooden structures approaching 100 feet in height were used very frequently, and 

 portable steel towers of the same height were used occasionally. It is with these tall 

 wooden structures that 272 and 2721 deal. 



Infrequently, signals have been constructed for use in triangulation to heights of 

 200 feet or more, but these have not been to specifications as each one has been more or 

 less of an individual case. These range from targets erected in the tops of tall redwood 

 trees to towers made from bamboo poles. Other organizations have used various 

 ingenious methods of erecting tall signals, one of which appears to have particular merit. 

 The French have mounted clumps of cabbage palm leaves on the tops of tall tubular 



