CHAPTER XV 



TRUE BEARmG 



By Theodolite — By Sextant — Variation. 



In nearly all descriptions of surveys true bearings will be 

 used. 



The most correct method, from a shore station, is to use By Theo- 

 the theodolite, which will alone give a very good result for sextant^ 

 azimuth ; but it is better to get the altitudes w^th a sextant 

 and artificial horizon, when two observers are available. 



The theodolite in this case is only used for taking the hori- 

 zontal angle between the sun and the zero. 



There are tliree principal methods in use for obtaining the Three 

 azimuth. By observations at equal altitude a.m. and p.m., by M®t^°ds 

 observations a.m. and p.m. at nearly the same altitudes, or 

 by single observations. 



The former is theoretically the more correct, as many errors Single 

 are eliminated ; the second is nearly as good ; but our experi- ggne^'jfy 

 ence is that with single observations taken with the sun near Sufficient, 

 the prime vertical, with instruments in good order, the result 

 is quite as near the truth as is generally requisite in marine 

 surveys. When an extensive piece of coast is being surveyed, 

 we shall, as before stated, depend partly upon the astronomical 

 positions for the scale and bearing of the chart ; but, neverthe- 

 less, accuracy in obtaining the original bearing for working is 

 necessary. 



A very important point is careful levelling, which should be 

 done wdth the telescope pointing in the direction of the sun, 

 and the accuracy of the movement of the telescope in a 

 vertical plane should be tested, as no method will entirely 

 eli^minate error due to want of such accuracy. 



In the first method, the sun will be observed at an even 



cJol 



