June 20, 1919] 



SCIENCE 



581 



ENGINEERS NEED ACCIRATE MAPS 



The contoured map must be of such ac- 

 curacy as to enable the highway engineers, 

 and engineers engaged on irrigation projects, 

 to lay out their work accurately. It can be 

 readily seen that with an accurately contoured 

 mail, the engineer can plan the railway, the 

 highway, etc., from one place to another, and 

 not make great mistakes in grades and align- 

 ment. It is doubtful if, even after consider- 

 able research work, airplane photography would 

 ever produce a map contoured accurately 

 enough for such engineering work. It is of 

 course possible that some method may be dis- 

 covered by which the differences in elevation 

 between two points shown on each of two sep- 

 arate photographs can be computed, but if one 

 considers that the work involved, if it can be 

 done at all, will be very great, he will see that 

 it will probably be more economical to put the 

 contours on the map by the usual methods, than 

 to compute innumerable elevations from photo- 

 graphs. 



The possible method of computing distances 

 and elevations from photographs may be sup- 

 plemented by using the stereoscopic method 

 which would give one an idea of the con- 

 figuration of the ground. This would enable 

 the draftsman in the office to select critical 

 points whose elevations could be determined. 

 Such critical points would be crests of hills or 

 ridges and the bottoms of slopes. If the ele- 

 vations of critical points are determined then 

 contours could be interpolated between them. 

 I am giving these statements with a good 

 deal of reservation on my part, for the method 

 of contouring by airplane photography has not 

 been developed and it may be that very little 

 can be accomplished where accurate contour- 

 ing is desired. Investigation has not yet been 

 carried to the point where one can state defi- 

 nitely the possibility or impossibility of con- 

 touring by this method. 



AIRPLANE SUPPLEMENTS PLANE TABLE 



But this accurate large-scale contoured map 

 can undoubtedly be made by combining the 

 usual methods of surveying with the aero- 

 photogpaphs. The aero-photographs will 



usually give a great deal of detail which may 

 facilitate the progress of the map by the topog- 

 rapher using the plane table. It will be nec- 

 essary of course for the topographer to select 

 a nimiber of definite points on his map, such 

 as road crossings, large buildings, groups of 

 buildings, bridges and other features which 

 can be identified from the photographs. Those 

 features would serve as control points for the 

 topographic details shown on the photographs. 

 Without such points located by the usual 

 methods, it would be necessary to place cer- 

 tain conspicuous objects on the ground near 

 the triangulation and traverse stations. Al- 

 most any kind of object that would show in the 

 photograph, and have a distinct shape, could 

 be used. But the placing of these objects 

 would be e.xijensive. It is believed that the 

 location of the conspicuous features referred 

 to above could be done by the topographer 

 at a much smaller cost than the cost of 

 placing objects for the aero-photographs, at the 

 triangulation and traverse stations. 



It is possible that the topographer would be 

 able to place the topographic details on his 

 map from the photographs before going into 

 the field to do the contoiu-ing. Much of the 

 work of the topographer by the usual methods 

 consists in placing the topographic features on 

 the map in their proper location, but a great 

 deal of this might be obviated by the use of 

 the photographs. Then he could go into the 

 field and place the contours with greater 

 rapidity than if he attempted to do so pre- 

 vious to using the details of the photographs. 



AIRPLANE VALUABLE FOR MAP REVISION 



What I have stated above in regard to 

 original surveys by aero-photography, in the 

 three classes of high-grade maps, are simply 

 opinions or prophesies. These are tlie coast 

 charts, the contoured maps of the interior, 

 and maps which show all features except con- 

 tours, but I feel confident in stating that 

 even on the highest grade of topographic maps, 

 the aero-photographs can be used to a great 

 degree in revising and bringing up to date 

 maps of that character which have already 

 been made. Let us suppose that we have be- 



