90 
HINTS TO TRAVELLERS, 
known. Time-measurement will be found most convenient, and, with 
care, will give very good results. Compass bearings need only be taken 
at every second station on the line of march. Objects on either hand 
should, where possible, be fixed by three bearings. It is not desirable 
to take compass-bearings to points more than 6 or 7 miles distant, as the 
prismatic compass can seldom be depended upon to within one degree, 
and an error of this amount in 6 or 7 miles would give an error of *05 inch 
on a scale of 2 miles to the inch. If the route runs near a peak, of 
which the true bearing has been determined from A, it should be as¬ 
cended, and a round of angles taken with the sextant, making A the 
zero-point. When there is a mid-day halt, the meridian altitude of the 
sun should be observed. If a field-sketch cannot be kept up, the route 
should be entered in a field-book, and afterwards plotted, before details 
are forgotten. A book—with every alternate page ruled into squares by 
strong lines, and subdivided by finer lines, the smaller squares represent¬ 
ing five minute intervals of time, the larger ones one hour-will be found 
of great use in making a rough sketch of the route; or a modification 
of the form used in booking a traverse-survey may be adopted. In all 
cases the bearings, distances, &c., should be clearly written in the book. 
In this field-sketch the ground has been treated as a plane surface, and 
as soon as convenient the work should be transferred to the projection on 
the fair plan. In doing this it becomes necessary to calculate the latitudes 
and longitudes of the camps, and other points, from the material provided 
by the survey; when this has been done, the fixed points are laid down in 
their true positions on the map, and the detail reduced to the proper 
scale. 
2. District Survey .-—The basis of any survey of an extensive district 
should be a system of triangulation, and the first step is the measurement 
of abase line. With no instruments except a sextant, tape and prismatic 
compass, the best plan is to measure an astronomical base, and thence 
extend the triangulation as far as may be necessary. Two suitable points, 
A and B, lying nearly north and south of each other, are selected as the 
ends of the proposed base; the position of A on the earth’s surface is 
determined at the point itself, the true bearing of B from A is obtained, 
and B having been made zero, a round of angles is taken with the sextant 
to conspicuous points; camp is then moved to the vicinity of B, and 
observations for latitude made at that point; the true bearing of A from B 
