448 proceedings: philosophical society 



The speaker discussed the question of whether accurate maps can 

 be made from aeroplanes and expressed the opinion that undoubtedly 

 aeroplane photographs can be used in the revision of existing maps. In 

 many cases where the country is practically flat, aeroplane photographs 

 may be used for original surveys. It is probable that difficulty would 

 be experienced in making contour maps from aeroplane photographs. 

 This would be due to the difficulty of having the camera in a vertical 

 position at the time the photograph was taken and on account of the 

 condition which would make it impossible to do accurate contouring 

 when the contour interval is small. Undoubtedly the aeroplane will 

 have considerable use in surveying and at present several of the govern- 

 ment organizations engaged in mapping are investigating the subject 

 of surveying from aeroplanes. 



Discussion. — Capt. Ellis spoke of the German and French maps 

 of eastern France. Mr. J. F. Hayford discussed the possibiHties of 

 mapping from aeroplane photographs. Mr. G. K. Burgess spoke of 

 the work of the French in constructing maps for Algeria and Morocco 

 from aerial photographs. 



The second paper was by Mr. Oscar S. Adams on A study of map 

 projections in general and was also illustrated by lantern slides. 



The difficulty encountered in map construction arises from the fact 

 that the earth's surface is ellipsoidal in shape and is consequently 

 non-developable, that is, it cannot be spread out in a plane without 

 some stretching, some tearing, or some folding. The determination 

 of a projection consists in fixing upon some system of lines in the plane 

 that will represent the meridians and parallels upon the earth. An 

 orderly arrangement of these lines, such as to give a one-to-one corre- 

 spondence between the points on the earth and those upon the plane, 

 is generally expressed in terms of some mathematical formula, and in 

 fact all projections in use can be so stated. 



In studying projections there are four main things to be considered. 

 These four considerations are: 



(i) The accuracy with which a projection represents the scale along 

 the meridians and parallels. 



(2) The accuracy with which it represents areas. 



(3) The accuracy with which it represents the shape of the features 

 of the area in question. 



(4) The ease with which the projection can be constructed. 



The scale of a map in any given direction at any point is the ratio 

 which a short distance measured upon the map bears to the correspond- 

 ing distance upon the surface of the earth. 



The subject of map projections is generally treated under the follow- 

 ing subdivisions: 



(a) Perspective or geometrical projections, (b) Conical projec- 

 tions, (c) Equivalent or equal area projections, (d) Conformal pro- 

 jections, (e) Azimuthal or zenithal projections. 



These classes are not, however, mutually exclusive since a given 

 projection may belong to two or sometimes three of the classes. 



