86 THE FORMATION 



where the topography is level, as sometimes happens in mapping single 

 formations, the chain and pedometer must be used to ascertain the size of 

 the different areas. Indeed in all mapping of vegetation, the methods of 

 surveying are directly applicable. Over large areas, however, it is not neces- 

 sary that limits be drawn with mathematical accuracy, and for the purposes 

 of the ecologist, the plane table and camera are satisfactory substitutes for 

 the surveyor's transit, at least in the present aspect of the subject. When 

 the formation or group of formations is commanded by an elevation of some 

 height, the latter is used as a base. A plane table is established upon it and 

 the topographical and vegetational features are recorded in the usual way. 

 This map is usually supplemented by a series of views from the same base. 

 Indeed it has come to be recognized that a complete series of photographs 

 of this kind give a more valuable record than the plane table, and that the 

 construction of an accurate map from them is an easy matter. Since the 

 camera saves much time and energy also, it is used almost exclusively to 

 furnish the data for map making. In hilly, and especially in mountainous 

 regions, the photographic method is indispensable. Its application is ex- 

 tremely simple. A central hill or mountain is selected, and from it a series 

 of views is taken so that the edge of one exactly meets the edge of the other. 

 This is an extremely important matter, and demands much nicety of judg- 

 ment. The camera is kept in the same spot, and after each exposure it is 

 turned as the operator looks through it until a landmark at one edge just 

 passes from view at the other. As soon as the new position is determined, 

 the tripod screw is turned to hold the box firmly in position. In case of a 

 slight jar, the exact position should again be obtained. If the series is 

 accurately made, the resulting prints will give a complete panoramic view 

 of the region, without overlap or omission. For this purpose, a Gj'^ x 8^^ 

 camera is desirable, since the topographic and vegetational features are 

 larger and stand out more distinctly. A large camera requires fewer changes 

 of position, and hence saves time and reduces the chance of error. A 

 4x5 camera serves the purpose sufficiently well, though it requires a little 

 more care in operation on account of the greater number of exposures neces- 

 sary. This may be avoided in some degree by the use of a wide-angle lens 

 if the depth of the area is not too great. Whatever camera may be used, a 

 telephoto lens is a very desirable adjunct, since it enables one to choose be- 

 tween three different sizes of the view without changing the position of the 

 camera. To avoid possible confusion, the exposures are always made from 

 right to left, and the plates are used in the numerical order of their holders. 

 For the same reason tlie landmarks are described and numl^ered in their 

 proper order. The prints obtained are mounted on a card in sequence. 

 The view map may be preserved in this form, or it may be reduced or 



