SURVEYING THE OCEANS (HYDROGRAPHIC) 29 



Photogrammetry is the best method of obtaining the topographic 

 information required for nautical charts and has replaced the 

 ground surveying method used almost universally just a few 

 decades ago. The aerial photograph is also the best source for 

 relative positioning of coastal and offshore detail, island groups, 

 and other terrestrial or marine features visible on the photograph. 

 Whereas black and virhite or panchromatic photographs were used 

 almost exclusively until recently, the newly discovered advantages 

 of using color and infra-red photography for special purposes have 

 added a new dimension to photogrammetry. Additional hydro- 

 graphic, topographic, and oceanographic data can now be recorded 

 on film for mapping, charting, and other scientific purposes. 



In the field of hydrography, the use of aerial photography 

 extends from planning for oceanographic survey operations to 

 determining water depths if water transparency and bottom con- 

 trast permit. It is anticipated that color aerial photographs, as 

 well as satellite photography, will play an increasingly important 

 role in the more rapid and comprehensive charting of coastal 

 hydrography. 



SURVEYING THE DEPTHS (BATHYMETRIC SURVEYING) 



Controlled surveys of the oceans at great distances from shore 

 are extremely expensive, and such precise, detailed surveys are 

 necessarily limited to special high priority projects vital to the 

 country's defense. Many gaps in our knowledge are being filled, 

 however, by other means. 



Present knowledge concerning the topography of the sea 

 floor — considerably greater than that of only a few years ago, 

 although admittedly less than desired — has been gained only 

 through the close cooperation of many organizations and inter- 

 ested observers. Ships of the Navy, the Military Sea Transportation 

 Service, the Coast Guard, and the merchant marine annually 

 contribute millions of miles of sounding data in the form of a 

 continuous bottom profile of their tracks throughout the world. 

 Evaluating these, comparing them with other tracks of the same 

 area, and integrating them with knowledge from precise surveys 

 are continuing tasks necessary for producing and maintaining 

 the best possible charts of the oceans. The Naval Oceanographic 



