38 THE U. S. GEOLOGICAL SURVEY 



tion. Bench marks also are of value in the progress of the 

 Survey's topographic work as fixed points of reference for the 

 extension of its level lines. 



It will be noted that the act of 1896 does not require that 

 the elevations of the bench marks above sea level be deter- 

 mined, but only "the elevations above a base level located in 

 each area." Any attempt to determine elevations above sea 

 level in the initial survey of an area might have necessitated 

 the running of thousands of miles of precise levels in order 

 to connect with sea level the initial points within each of the 

 areas under topographic survey. This would have cost im- 

 mense sums and would have occupied several years, during 

 which the topographic surveys would have had to be con- 

 tinued without spirit leveling. The Survey, therefore, desig- 

 nated some point within each area under topographic survey 

 as a central datum point for that area, and determined the ele- 

 vation of that point as nearly as practicable from existing ele- 

 vations adjusted by railway levels or other levels brought 

 from the sea. In consequence, though all the elevations re- 

 ferred to the same central datum point agree one with the 

 other, they may not be reduced to mean sea level because of 

 the differences between the primary elevations on which the 

 leveling is based. However, prior to 1 and since 1896 precise- 

 level lines have been extended by the Survey and other organi- 

 zations to more accurately determine inland elevations above 

 sea level, and the elevations originally determined have been 

 corrected from time to time, so that at present nearly all the 

 central points have been reduced to mean sea-level datum and 

 carry with them all the levels resting thereon. As the area 

 referred to a single base is gradually widened, it becomes pos- 

 sible to correct the elevations with reference to a new base, 

 the corrections being, if practicable, entered upon the bench 

 mark. 



The location, and elevations of the bench marks are pub- 

 lished in two ways— first, by a symbol on the atlas sheets, ac- 

 companied by the letters "B.M." and the elevation to the near- 



