TOPOGRAPHIC SURVEYS. 5 
extent of the journey. ‘The essential object, an adequate and expressive 
contour map, was not lost sight of, but economy of time and extension 
of the work over the largest possible area were requirements constantly 
imposed. A method of graphic triangulation with the plane-table was 
adopted as the most satisfactory, wherever conditions permitted its use, 
By this method triangulation is expanded from a measured base line by 
sights taken with a telescopic alidade and recorded simply by a pencil 
line drawn upon the precisely oriented plane-table sheet. The intersec- 
tion of three or more fine lines in a single point is taken to indicate the 
position of the object sighted, within the limits of the scale of the map. 
The method and the results which may be attained by it are fully described 
in the manual already referred to.* Vertical angles are measured with 
the telescopic alidade, and from these differences of elevation are calculated. 
The altitudes of all primary points are determined as the average of 
numerous observations from as many different stations. Those of minor 
features which are required for controlling contours are calculated at once 
on the station from the observed angle and known distance, and the con- 
tours are indicated on the sheet in their proper positions. In doing this, 
care is taken to draw them so that they shall express the form of the 
observed feature, whether angular or rounded, precipitousor gently sloping, 
and thus convey a correct impression of the character of the surface. In 
such rapid work as was executed in China it was not usually possible to 
connect up all the contours and complete the map in the field. The final 
drawing was made in the office after our return, on the basis of the features 
sketched in the field and with the aid of numerous photographs. 
As already stated, graphic triangulation was not always practicable, 
and we resorted to stadia surveys. The methods and precautions used in 
this class of the work are set forth in connection with the sections thus 
surveyed. 
The description of the surveys which follows falls into several sections 
according to the districts, the conditions, and the methods. The nature 
of the country passed through is illustrated in the plates accompanying 
this chapter, which are especially selected to show the very distinct types 
of hill and mountain, valley and canyon of North and South China, and 
the wide difference between the treeless districts of the north and the 
wooded region of the south. 
z Chapter VII, Article 54 and Chapter [X, Article 74 

