578 



SCIENCE 



[N. S. Vol. XLIX. No. 1277 



nitely each area within ■which there are new 

 features or where old ones have changed. It 

 is doubtless true that in such cases the details 

 of an airplane photograph could be placed on 

 the map from the photograph with all of the 

 accuracy that is needed in the topography 

 shown on the coast charts. 



It is a debatable question as to whether the 

 airplane photograph made over a water area 

 will show any outline of submerged dangers 

 to navigation when the plates are developed. 

 If they do there is a vast field for the air- 

 plane in making photographs over water areas 

 where it is known that many obstructions 

 exist. With the usual surveying methods, it is 

 difficult at times to locate every obstruction. 

 One or more on any chart might be missed. 

 This fact has been proved a number of times 

 in a most disastrous way by vessels running 

 on uncharted rocks both along our eastern and 

 our we|Stem coasts and especially in Alaska. 



The Coast and Geodetic Survey is now ma- 

 king wire drag surveys of all doubtful areas 

 along the coast, but it will be many years be- 

 fore the bureau can assure the navigating 

 public that all obstructions have been found 

 and accurately charted. 



.URPLANES AND HYDEOGEAPHIO SURVEYS 



It m.ay be possible that an airplane photo- 

 graph will indicate submerged rocks or other 

 dangers that are close to the surface of the 

 water. It would be a question of differences in 

 shade in the photograph. If such a detection 

 of danger can be made then it will be nec- 

 essary to make the photographs only on per- 

 fectly clear days. Otherwise, the shadow on 

 the water of a passing cloud might show on 

 the photograph and cause uncertainty as to 

 whether the spot was a cloud or an actual ob- 

 struction. 



There are many hundreds of square miles of 

 area along the coast that consist of salt 

 marshes with many streams of little or no im- 

 portance, but which should be shown in their 

 proper relation to other topographic features. 

 These marshes can be photographed from air- 

 planes and the streams running through them 

 would probably show in such a way that they 



could be fitted into the map from the photo- 

 graph. Here might be a large saving of time 

 for the surveyor in the field. 



There are other cases where there are exten- 

 sive mud flats, when the tide is low, as in 

 Jamaica Bay, New York. To survey the out- 

 line for these flats is rather laborious, with the 

 usual instrumental methods, but it is believed 

 that it might be possible to photograph them 

 from an airplane and have the results placed 

 on charts. It will undoubtedly be possible to 

 get these located on the charts from airplane 

 photographs with all the accuracy that is nec- 

 essary for the navigator. 



LOCATION OF DANGERS TO NA-VIGATION 



In making photographs from the water, for 

 the purpose of discovering obstructions to 

 navigation at low tide it will be necessary to 

 have some means to properly locate photo- 

 graphic features on the chart. This probably 

 can be done by anchoring two or three small 

 boats within the area of the photograph and 

 locating them with relation to triangulation 

 stations. The location could be done in the 

 usual way in which the sounding boat is to- 

 day given its position, that is by taking two 

 sextant angles simultaneously from the boat to 

 three control points. It can readily be seen 

 that if two or three accurately located boats 

 are clearly shown on the aeroplane photo- 

 graph, it will be possible to place the topo- 

 graphic details on the map in their proper 

 positions. 



I do not wish to convey the idea that the 

 airplane photography will supersede the usual 

 methods of hydrographic surveying, but it 

 would supplement those methods by making 

 it possible to discover channels running 

 through mud flats, also coral heads, shoals, 

 and other obstructions which might be close 

 to the surface of the water and which may be 

 missed by the usual methods of conducting 

 hydrographic surveying. 



There has been a rather positive statement 

 made above that the airplane can be used to 

 advantage in the work of the Coast and Geo- 

 detic Survey. This is undoubtedly true, but 

 only time and the development of the methods 



