271 HYDROGRAPHIC MANUAL ' PaGE 152 



described in chapter 6, an indirect method which does not depend on the visibihty 

 of objects. 



For the first method either a sufficient number of natural objects must exist or 

 artificial objects must be constructed. It is with the construction of these latter, 

 signals and buoys, that this section and section 28 deal. In the Coast and Geodetic 

 Survey, the term signal is used loosely to indicate any sort of artificial object erected 

 or established on the land for use in measuring sextant angles to locate the positions of 

 survey launches and vessels engaged in sounding. A signal may be a white washed 

 rock, a 100-foot tower, or a handkerchief-sized banner on a bush. The term is some- 

 times even used to indicate a natural object used for control. (See 2111.) The term 

 buoy is likewise used loosely to indicate any anchored floating object, with or without 

 superstructure, used for control purposes. 



It is obvious that hydrographic surveying is greatly facilitated when the objects 

 at the control stations are conspicuous enough to be seen readily by the observers. 

 For this reason, as well as for economy and durability, natural objects such as lone 

 boulders, pinnacle rocks, waterfalls, lone trees, and distinctive marks on cliffs, and 

 artificial objects such as tanks, lighthouses, spires, and building gables, should be used 

 for control wherever available. 



Signal structures erected at stations vary in type and size, depending on their loca- 

 tion, purpose, and the materials available. Their size and elevation above high water 

 depend on whether they are to be used for launch or ship hydrography and at what 

 distance they need to be visible. Advantage should be taken of natural elevations of 

 the ground upon which to erect signals, where they need to be seen at considerable 

 distances. 



Where a signal is to be used at a long distance, the target not only must be of con- 

 siderable size but must be constructed and dressed so that it will stand out in contrast 

 to the background and be as conspicuous as possible. In this respect white is best 

 against a dark background and black is best against the skylme, provided the target 

 is elevated sufficiently or constructed so as not to blend in with ground objects. 

 When white targets are used, they should be constructed to reflect as much sunlight 

 as possible. The principal advantage is to be gained from a white target only when 

 the sun shines on it, and one which must be pointed away from the sun can be seen 

 ordinarily only from comparatively short distances. But under certain conditions, 

 a white target illuminated by the sun on its inshore side will stand out conspicuously 

 against a nearby dark background of trees or cliff. A sloping target will reflect the 

 most sunlight and consequently will be visible at the maximum distance. The slope 

 should be from 45° to 60° and the signal should be constructed and dressed in accordance 

 with the conditions encountered. A tripod structure dressed with white signal cloth 

 to resemble a 9- by 9-foot tent of the centerpole type makes an excellent signal. 



Where shore signals are needed to control offshore ship hydrography in areas 

 where the coastline is low and flat, targets have to be placed on high structures to be 

 visible from any appreciable (fistance offshore. To obtain better reflection from the 

 sun, it is sometimes advisable to orient the signal and target at a slight angle to the 

 shoreline, rather than have it face directly offshore. This angle should not exceed 15°. 

 To construct the target at an angle to give it greater reflecting power, the target boards 

 can be shimmed at the bottom to give the target a Venetian-blind effect, and can be 

 slightly sprung around the tower to present a convex surface to seaward. 



For the control of launch hydrography, it is seldom necessary to construct signals 

 at any appreciable elevation above the high-water line. Almost any type of signal will 



