loo 
HINTS TO TRAVELLEBS. 
should then be placed accurately on the line previously drawn to indicate 
its position, as before described, and the plane-table turned round in 
azimuth until the needle points to 0°, and then clamped. 
Three fixed points should then be selected from which to interpolate 
the position. The points should be as near as possible arid chosen so 
that the observer is inside the triangle formed by joining the three points. 
The ruler is then laid on each point in succession and lines drawn along 
its edge. If the plane-table has been set up accurately in azimuth, the 
three rays will intersect in a point, which is the required position. More 
frequently, however, the intersections form a small triaegle of error, in 
which case it is necessary to determine the true position. 
First, where the observer’s position is inside the triangle formed by 
joining the fixed points. In this case the true position will be within 
the small triangle of error formed by the intersection of the rays. It 
will also occupy such a position that its perpendicular distance from each 
ray will be in proportion to the distance of the observer’s position from 
the respective trigonometrical points. 
Thus in Eig. 2 , p will be the correct position, the perpendicular dis¬ 
tances p a,.p b, p c being proportional respectively to p A, p B, p C. 
Secondly, where the observer has been forced to use three trigono¬ 
metrical points so placed that his position lies outside the triangle formed 
