April 1, 1921] 



SCIENCE 



309 



garding their use for hydrograpliic purposes 

 were considered. Obviously, a comparison 

 was made with, the present-day methods of 

 hydrographic surveying. 



It may be argued that aerial photography 

 is more rapid, because a photograph of more 

 than one square mile is made in a fraction of 

 a second, and a strip 70 miles long and over 

 a mile wide can be photographed in an hour. 

 There are several problems to be overcome by 

 both the aviator and the hydrographer before 

 this can be done. Weather conditions along 

 the sea coast are not as suitable for aerial 

 photography as might be expected. Let us 

 see how the photographs as made by the 

 French would apply to our waters. These 

 photographs were made under the following 

 conditions: Focal plane horizontal; altitude, 

 2,600 meters; at time of low water; the sun 

 high above the horizon; calm sea. Along the 

 coast of the United States, a calm day is gen- 

 erally hazy, so much so that it is impossible 

 to make photographs from an altitude of even 

 4,000 feet without special treatment of plates 

 or films. We are aware of recent experiments 

 regarding the penetration of haze, but at the 

 time the Key West experiments were made, 

 little was known of this new process. Further 

 developments may make it possible to pene- 

 trate haze at altitudes of 2,600 meters. But 

 disregarding haze, those days that are calm 

 and cloudless are infrequent. It is difficult 

 to obtain data regarding meteorological con- 

 ditions as affecting aerial photography along 

 the coast, but from available data, it is ven- 

 tured that about one day a month would ful- 

 fill conditions as called for by the French, 

 and that is believed to be an optimistic esti- 

 mate. 



Regarding control for the photographs, very 

 few places along our coast are as ideally fitted 

 for control of aerial protographs as the area 

 chosen near Brest. This locality is dotted 

 with numerous small islets, and ample control 

 could be obtained for each photograph. At 

 Key West, it was necessary to use boats as 

 control points, so that the speed at which an 

 area was covered was limited to the speed of 

 the vessels. There are a few places along our 



coast where enough land stations would ap- 

 pear for control, but these areas are generally 

 in bays or rivers where water is not clear 

 enough for good photographic work. Buoys 

 or rafts may be used as control points, but 

 the cost and labor of handling them would 

 be excessive. A raft about 10 feet in diameter 

 would be needed in order to be legible on a 

 1:10,000 scale photograph. The problem of 

 handling a large number of these floating 

 signals would require a good sized vessel and 

 crew. 



The uncertainty of results is another factor. 

 The French have solved some of the problems 

 by using the stereoscope, so that the confu- 

 sion, brought about by vari-colored bottom of 

 uniform depth, is partly eliminated. Some 

 shoals will show clearly, while others close by 

 do not appear in the photograph, probably due 

 to a difference in color or lighting. The 

 photographs will not record all shoals as seen 

 by the aviator. It is often necessary to fly 

 over the same area repeatedly in order to ob- 

 tain good results. 



Unless ideal conditions prevail, the cost of 

 an aerial survey with present-day equipment, 

 will far exceed that of a wire drag survey, 

 and will not give as certain results. We be- 

 liove that aerial photo-hydrography is of some 

 use in a few limited locations, and there are 

 possibilities of future development, but at the 

 present date, revision work by photographs on 

 land holds forth greater promise, and is one 

 in which more certain results can be obtained. 



It may be of interest to quote a sentence 

 from a letter dated January 10, 1921, from 

 Le Directeur du Service Hydrographique ad- 

 dressed to the Director of the Coast and Geo- 

 detic Survey, in which the following state- 

 ment is made regarding aerial photography 

 along the coast of Syria in 1920. 



Les circonstances n'ont d'ailleura pas permis de 

 1 'employer systematiquement. (The circumstances 

 do not, however, permit of its systematic use). 



A careful analysis of the conclusion reached 

 in the article " Siirveying from the Air," espe- 

 cially of the qualifying words " using present- 

 day equipment," and " little practical value," 



