CHAPTER III 



BASES 



By Chain— By difference of Latitude— By Mast-head Angle— By Angle sub- 

 tended by known length — By Range-finder — By Measured Rope — By 

 Sound. 



DiflFerent Bases for marine charts or plans upon which to build the 

 Bases °* triangulation are obtained in several ways, according as cir- 

 cumstances permit and accuracy requires. 



1. By means of the 100-feet chain or steel tape supplied for 

 the purpose. 



2. By difference of latitude, or difference of longitude. 



3. By measuring with a micrometer or sextant the angle 

 subtended by a known length, as two poles a measured distance 

 apart, the ends of a long pole, or the masts of a ship. 



4. By range-finder. 



5. By a measured rope, as a lead-line ; or by the wire from 

 a sounding machine. 



G. By sound. 



CHAINED BASES. 



The ground for a base, to be measured either by chain or 

 rope, must be as level as can be found. Its length will be 

 partly determined by the extent of the work to depend on it, 

 varying from say 9,000 feet to 1,000 feet, or even less for a 

 small harbour. 



While it is certainly convenient to measure the base in one 

 straight length, if convenient ground can be found, it is by 

 no means necessary. 



If several short lines making angles with one another are 

 measured with the angles carefully observed, the terminal 

 points being visible from one another, the resulting distance 



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