August 17, 1917] 



SCIENCE 



165 



There is scarcely any surveying or civil 

 engineering -which does not require that dif- 

 ferences in elevation be determined by spirit 

 leveling and in nearly all cases the absolute 

 elevation of the bench marks above some plane 

 of reference or datum is determined. Effi- 

 ciency in operation frequently depends upon 

 the datum selected. There are many other 

 branches of science besides that of engineering 

 in which absolute elevations are needed. 



The selection of a fundamental datum is a 

 matter of great importance. Only slight con- 

 sideration leads one to conclude that the ideal 

 datum for a nation is one which may be estab- 

 lished at many places. The only one of this 

 kind is mean sea level. 



Mean sea level may be established within a 

 very small fraction of a foot by continuous 

 tidal observations for at least a year. It has 

 been found from precise leveling observations 

 that mean sea level, as established at different 

 points on the open coasts, is at all such points 

 in the same equipotential surface; that is, if 

 there were no resistance of the water and wind 

 to the movement of an object floating on the 

 ocean, the object could be moved from one 

 point on the coast to another without perform- 

 ing any work — there would be no lifting neces- 

 sary. While this statement may not be abso- 

 lutely true, yet it is so nearly the case that for 

 all engineering and surveying purposes it may 

 be accepted as rigidly true. 



Mean sea level is used exclusively in the 

 work of the Coast and Geodetic Survey and 

 the TJ. S. Geological Survey. It is used to a 

 certain extent by many other engineering 

 bureaus of the government. 



In December, 1916, the Coast and Geodetic 

 Survey sent the following letter, or one similar 

 to it, to the chief engineers of most of the 

 large cities of the country, to the State Engi- 

 neer of each state, and to the chief engineer of 

 each of about 150 railroads in the United 

 States : 



As you know, one of the important questions of 

 the United States Coast and Geodetic Survey is 

 the extension over the country of a network of 

 precise leveling which will give elevations of great 

 accuracy, based upon mean sea level. 



We believe that this precise leveUng is essential 

 in the surveying and engineering work done in this 

 country by various public and private agencies. 

 The network will enable engineers to use the sea- 

 level datum on new projects and to reduce to this 

 datum existing elevations referred to arbitrary 

 datums. We believe that this country should 

 eventually have but one datum, in order that all 

 engineering and surveying work may be easily 

 coordinated. We believe also, that the presence 

 of various datums leads to much confusion and 

 waste. 



In order that we may get into closer touch with 

 the needs of the engineering profession, I should 

 be glad if you will let me know to what extent 

 your state is basing the elevations of its road and 

 other surveys and engineering works upon mean 

 sea level; also whether the use of various arbitrary 

 datums by coimties, cities and private organiza- 

 tions within your state is a serious matter in the 

 industrial development of your state. 



Eeplies were received from many of the 

 engineers to whom the above letter was written. 

 The opinions expressed were almost unani- 

 mously in favor of the adoption of mean sea 

 level as the datum for elevations. 



The pamphlet under discussion contains 

 quotations from many of the letters received 

 by the Survey. One of the quotations, tjijical 

 of most of them, reads: 



So far as our experience has taught us there can 

 be no question as to the desirability of a universal 

 datum plane, and I think there can be no doubt in 

 the minds of engineers engaged in municipal work 

 that mean sea level is the only logical datum to 

 adopt. 



In your advocacy of an extension of such bench 

 marks you deserve the support and cooperation of 

 every engineer in the country. 



Another reads : 



We agree with you that it would be very valu- 

 able to the state if a system of levels could be es- 

 tabUshed, and believe that such will need to be 

 done in the near future in order to correlate the 

 drainage, highway and other engineering work in 

 the state. 



It is realized by the members of the Coast 

 and Geodetic Survey that much of the con- 

 fusion in datums which now exist, is due to 

 the fact that the precise level net of the United 

 States was not extended in the past as rapidly 



