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The Secondary Base in Geodetic Surveys * 



By O. B. Wheeler. 



In geodetic surveys three systems or grades of triangulation 

 are recognized, each having its distinctive base. 



The primary system has bases from four to six miles in length, 

 which are measured with the greatest accuracy attainable. 



Usually, these bases are from 200 to 400 miles apart, and are 

 connected with lines of triangulation as long as possible compati- 

 ble with well-conditioued triangles. 



The bases of the secondary system, which we wish to consider, 

 are from li to 2 miles in length, usually from 80 to 100 miles 

 apart, and the lines of triangulation are from 2 to 15 miles in 

 length. 



The tertiary system has bases from from i,ooc to 2,000 feet in 

 length, usually 10 to 15 miles apart. 



In quality, the primary base is too expensive both in the cost 

 of apparatus and in the time required to make a measurement ; 

 and the tertiary base, which is a simple chain or steel tape meas- 

 urement upon the ground, is not sufficiently refined or accurate 

 for the secondary base. 



Not long since, the most approved method of measuring a sec- 

 ondary base was with wooden or glass rods on a rope made taut 

 over posts, the rods being grooved to half the size of the rope 

 and clamped by hand to the rope, while a rear rod was carried 

 to the front. The base was measured in sections. There was an 

 imperfect contact in rods, and one can imagine a very uncertain 

 amount of creeping and crawling of the rope base line under un- 

 conscious strain on the rope, given by the operators, and under 

 changes of sunshine and shadow, humidity of atmosphere, &c. 



* Read June 6tli, 18S7. 



