June 20, 1919] 



SCIENCE 



579 



can show just how much the airplane can be 

 used by this bureau. 



THE AIKPLAXE IS" TOPOGRAPHIC MAPPING 



We come now to the third class of maps and 

 that is a subject on which I hesitate to ex- 

 press an opinion. That is the mapping of the 

 interior of the country. This work is under- 

 taken by the U. S. Geolog-ical Survey, supple- 

 mented to a certain extent by the Corps of 

 Engineers, U. S. Army. The Coast and Geo- 

 detic Survey cooperates with those two or- 

 ganizations to the extent of furnishing the 

 fundamental horizontal and vertical control 

 for the surveys and maps, but almost all of the 

 actual location of artificial or natural features 

 is done by the other organizations. It is 

 understood that the officers of those two or- 

 ganizations have given consideration to the 

 question of map-making by airplane photo- 

 graphs. It is hoped that airplane surveying 

 can be developed at least to supplement the 

 usual surveyor's methods in mapping the in- 

 terior on a comparatively large scale map 

 with high accuracy. 



It would appear that if the airplane photo- 

 graph will be of so much assistance in the 

 topographic work along the shores of the coun- 

 try that it -would really be of some value in the 

 interior. 



"Whether or not it is possible to locate con- 

 tours from airplane photographs is a ques- 

 tion that has not yet been decided. Many per- 

 sons who have studied the question claim that 

 it is impossible to locate contours accurately 

 from airplane photographs. Others claim that 

 they can be located with great accuracy. The 

 substance of the situation is probably this: it 

 will be possible later to devise methods of con- 

 touring from photographs provided that we 

 can solve one or two of oui present more fun- 

 damental problems. It is possible that the 

 stereoscopic method can be applied to two 

 photographs taken by two cameras on the same 

 airplane or by cameras on two different air- 

 planes together to obtain a rough idea of the 

 configuration of the country. 



SURVEYING THE INTERIOR OF THE COUNTRY 



With regard to surveying the interior of the 

 country for the purpose of making an accurate 



large scale, contoured map, I may say that here 

 the airplane photographs can undoubtedly sup- 

 plement the usual surveying methods, but can 

 not entirely supplant them. 



Such a map should probably be on a 1/50,000 

 scale, that is one foot on the map would equal 

 50,000 feet on the ground, and the distance be- 

 tween control points on the opposite edges of 

 the area of a map should be correct within 

 about 1 part in 10,000. The only method by 

 which this can be accomplished is by tri- 

 angulation and transit and tape traverse. 

 The method to-day is to establish the triangu- 

 lation and traverse stations ahead of the topo- 

 graphic surveying, with the geographic posi- 

 tions, that is latitudes and longitudes com- 

 puted on the North American or final datinn. 

 When the control points are placed on that 

 datum their positions will not have to be 

 changed when two maps are joined. 



The control, namely triangulation and tra- 

 verse, bears tlie same relation to the topo- 

 graphic mapping of the country that the steel 

 framework of a sky-scraper bears to the de- 

 tailed portions of the building, such as walls, 

 floors, doors, windows, etc. If the steel work 

 is not accurately fastened and adjusted when 

 erected, before the detailed portions are 

 started on a building, it is reasonably certain 

 that the building will be distorted in shape 

 and will be structurally weak. 



The same idea pertains to maps, and the 

 difficulty mentioned actually exists to-day in 

 some parts of our country, where the detailed 

 mapping of certain areas had to precede the 

 triangulation and traverse based on the North 

 American datum. The result has been over- 

 laps, gaps, offsets, etc., when two maps, based 

 on different data have been joined together. 



LATITUDES, LONGITUDES AND ELEVATIONS NEEDED 



It is the province of the Coast and Geodetic 

 Survey to extend the fvmdamental control, 

 that is, latitudes and longitudes in long arcs 

 throughout the countrj'. These arcs are inter- 

 laced in order that the requisite strength may 

 be obtained. This work has been carried on 

 as vigorously as the funds at the disposal of 

 the Survey would permit. We have arrived 

 at a situation to-day which demands that this 

 work be expedited, and it is hoped that Con- 



