SEXTANT AND PRISMATIC COMPASS SURVEY. 91 
is then obtained, and a round of angles taken to the points previously 
observed to from A. The length of the base A B can then be computed and 
the position of several of the points observed to from A and B determined. 
The fixed points are next laid down on the field-sheet, and the detail 
filled in with the prismatic compass. In this way the triangulation may 
be extended over the district to be surveyed, care being taken to 
check the work occasionally by observations for latitude at selected 
points. 
The following notes and problems 515 will be found useful in constructing 
the map:—• 
Problem 7.—Let A and B be two stations visible from one another, 
AP = &, BP = a, their observed co-latitudes; the angles A and B their 
p 
B 
reciprocal true azimuths; and APB, or P, the required angular 
difference of longitude. Then by spherical trigonometry— 
which determines P. 
Problem IL —The latitude and longitude of any point being known, that 
of any other point within a short distance can be determined by plane 
trigonometry. Suppose the latitude and longitude of the camp at A to be 
known, whence that of a neighbouring peak or land-mark, 0, is to be 
determined; the distance A 0 must be measured, and the azimuth NCA 
observed, then the difference of longitude AB is the sine of A C B to radius 
* Problems II.-V. are taken from Frome’s ‘Outline of a Trigonometrical 
Survey,’ revised by Major-General Sir C. Warren, b,e. 
