( 75 ,.) 
PART III. 
SURVEYING-, 
Mapping a Country. 
The surveys that are mostly possible for travellers are route surveys, 
i.e., laying down as much of a country as comes within the ken of a 
traveller on his line of march. Such surveys, if of any extent, must be 
assisted by astronomical observations to prevent the accumulation of 
errors. (See pp. 82, 135.) 
Eoute surveying can be accomplished in several ways, but in any case 
is not an easy task for one who has no experience of ordinary surveying, x 
as, to be successful, it requires a knowledge of how to make the most of 
opportunities, of which method is applicable, and generally a mastery 
of the various dodges by which alone an irregular survey can be made 
to give a result fairly approximating to the truth. 
The principle underlying all surveying is to start from a base line of 
known length, and by means of angles or bearings to obtain rays to 
conspicuous objects from both ends, by the intersection of which their 
position can be fixed. Details are sketched in between. 
The base line may be long or short, may be measured, either accu¬ 
rately, by means of a tape, cord, chain, etc., by astronomical observations, 
by triangulation in the manner shown, pp. 90,120, 121, or, roughly, by 
estimation of the distance walked in a straight line. 
Tacheometer surveying is a method in which an extremely short base 
is used, the angle subtended by it at a point at right angles to the centre 
of the base being measured from the point to be fixed; in this case not 
at a great distance from the base. 
To aid the traveller, descriptions will be given of:—- 
(1.) Route surveying with Prismatic Compass, p. 76. 
(2.) Surveys with Sextant and Prismatic Compass, p. 87. 
(3.) Surveying with a Plane Table, p. 97. 
(4.) Surveying with a Tacheometer, p. 111. 
(5.) Surveying with a Theodolite, p. 116, 
(6.) Photographic surveying, p. 123. 
