S U R 



The following must only be considered a mere outline of the method 

 pursued in surveying a country : the niceties necessary to be attended 

 to, in crder to render such survey available for scientific purposes, can- 

 not be here described, 



2. To carry on a measurement by a 5erii?s of triangles. 



I^eta base line ABbe measured,* 

 and having fixed upon two objects C 

 and D, observe the /'s B A C, B A D, 

 ABC, A B D ; then in the A A B C, 

 the 's B A C, ABC, being known, 

 their supplement A C B is known, .'. 



A C and C B may be found by Case 1. 



Plane Trigonometry. The relative A B 



bearings and distances therefore of A, B, C arc thus deterirmied. Again 

 in the A A B D, the ^'s D A B, D B A being known, A D B is known, 

 and /. D B may be found. Lastly, in the A D B C, the sides B C, B D 

 and the included ^ C B D are known, .'. the remaining /'s B C D, B D C 

 may be found, and consequently also the side CD (see Case 2. Plane 

 Trigonometry). The bearings .'.and distances of B, C, D are also known. 

 In the same way, by considering either A C, C D or D B as a new base, 

 and fixing upon two other points ; the measurement may be continued 

 at pleasure. 



In conducting geodetical operations, the following rules by Hutton 

 should be observed, to diminish the probability of error. 



(1) When one side only of a triangle is to be determined, the measured 

 base should be nearly equal to the required side. 



(2) W T hen two sides of a triangle are to be determined, the triangle 

 should, if possible, be equilateral. 



(3) When the base cannot be equal to one or both the required sides, 

 it should be as long as possible, and the two angles attbe base equal, and 

 not less than 20 or 30 degrees. 



In the late survey of England the base first measured was upon Houn- 

 slow Heath. By continuing the measurement to Salisbury Plains, the 



* To reduce a base on an plevatpd level to that at the surface of the 

 sea, let r =. rad. of earth at the surface of the sea, r -f k the rad. referred 

 to the level of the base measured, the altitude h being determined by 

 the barometer, B the length of the measured base at the altitude h, then 



the correction is nearly, \vbu-h must bp subtracted from the mea- 



*urpd base, to give the true bafe at tho level of the eea. 

 i?88 



