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CONTEOL AND SIGNAL BUILDING 



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preventing- any inland triangulation stations from being visible from the shore. Also 

 the beach is so rocky and the shoreline so indented that measurement by taped 

 distances is impossible but, since the beach is not maccessible, control for planetable 

 topography can be established by offset traverse. 



Figure 11.— Where offset traverse can be used advantageously along the shore (between A and M). 



Figure 12.— The offset traverse method. 



The traverse is run along the outer coast of Onward Island between triangulation stations at A and M. The distances between 

 traverse stations are derived from taped offsets and distance angles. Referring to figure 12, triangulation station A is occupied with 

 a theodolite. Any of the other triangulation stations is used for orientation, and a direction angle for azimuth is measured to B, the 

 next forward traverse station selected. An offset station a is then established at exactly right angles to the line AB where it will be 

 visible from B, and about 30 meters distant from A. The angle aAB is then observed and the horizontal distance Aa is taped accu- 

 rately. Then traverse station B is occupied and directions to A, a, and C the next forward traverse station are observed. The 

 distance angle ABa combined with the 90° angle at .4 and the measured distance Aa are the elements needed to compute the slender 

 triangle AaB and to obtain the distance AB. At B then an offset station 6 is selected at exactly right angles to the line BC, the angle 

 bBC is observed and the distance Bb measured. The same procedure is followed in occupying subsequent traverse stations. The 

 90° angles to the offset stations should not be included in the same horizon closure with the other angles, if a repeating theodolite is used. 



With offsets always normal to the direction of the forward station, the logarithm 

 of the distance between traverse stations is the difference between the logarithms of the 

 offset distance and the tangent of the distance angle. 



If the horizontal distance to an offset station cannot be measured, measure the 

 inclmed distance and correct it for inclination. The difference in elevation between a 

 traverse station and its offset station may be determined by bringing the eye in the 

 plane of one station and the visible sea horizon and noting the depression of the other 

 station below the plane on a graduated pole. Another method of obtainmg the hori- 

 zontal distance is to stretch the tape between the horizontal axis of the telescope and 

 the offset station, measurmg the inclined distance accurately. Sight the telescope 

 along the tape to the offset station and measure the angle of elevation or depression. 

 From these data the horizontal distance is computed. 



