What Is Mean Sea Level? 



If there were no disturbing influences 

 the ocean would be of one equilibrium 



By William Bowie 



Chief of the Division of Geodesy, U. S. Coast and Geodetic Survey 



speaks 

 of the eleva- 

 tion of a 

 place, he has 

 in mind the 

 vertical dis- 

 tance above 

 an imagina- 

 ry surface. 

 The surface 

 is generally 

 that of the 

 oceans imag- 

 ined to ex- 

 tend inland 

 under the 

 point con- 

 sidered. 



If there 

 were no dis- 

 turbing in- 

 fluence by 

 the sun and 

 moon, the 

 force of the 

 winds and 

 the varying 



pressure of the air, then the surface of the 

 oceans at all places would be one of 

 equilibrium. If lines of precise leveling 

 were extended inland from the Atlantic, 

 Gulf and Pacific assumed to be quiet to 

 Chicago, for instance, and if the leveling 

 were done absolutely without error, the 

 elevations of their junc- 

 tion points would be 

 absolutely the same for 

 each line. 



But the waters of 

 the oceans are rising 

 and falling in response 

 to the forces acting on 

 them, so in order to 

 obtain an accurate 

 starting point, a tidal 

 station must be estab- 



How the Observations Are Taken 



The leveling is done along t he railroads because of easy transportation. 

 The engineers ride on motor velocipedes. Note the sunshade and wind- 

 brake. The leveling instrument, which was designed by an official of the 

 U. S. Coast and Geodetic Survey, is considered the most satisfactofi' one 

 in the world. Its tripod is mounted on one of the motor cars. On the 

 second car there is placed a listing adding-machine on which the 

 observations are recorded. Wonderful speed is attained. A party 

 working in Michigan in 1916 ran 340 single miles of levels in one month. 





1 :, 1,. !,. 1, II .. , ..... .1,1 111 



into masonary or concrete 



lished, and 

 the position 

 of the wa- 

 ter's surface 

 with relation 

 to a graduat- 

 ed staff must 

 be observed 

 each hour for 

 at least a 

 year. The 

 average po- 

 sition of the 

 surface of 

 the water 

 during the 

 year will be 

 almost ex- 

 actly mean 

 sea level. 



Under the 

 direction of 

 Major E.Les- 

 ter Jones, its 

 superintend- 

 ent, many 

 such stations 

 have been 

 established 

 along the coasts of the United States by 

 the Coast and Geodetic Survey, and lines 

 of accurate levels have been extended 

 from them to furnish bench marks, which 

 control the surveying, engineering and 

 mapping done by the Federal Govern- 

 ment, the States and cities and by pri- 

 vate persons. 



^ An elevation is the 



vertical distance above 

 what would be the sur- 

 face of the water if a 

 sea-level canal were ex- 

 tended inland from the 

 ocean to the bench 

 mark. 



Each line is run twice 

 in opposite directionsto 

 guard against errors. 



408 



