THE METAL, DISCS WITH WHICH THE COAST AND GEODETIC SURVEY MARKS ITS 



TRI ANGULATION STATIONS 



Each triangulation is marked by brass and iron discs as shown in the picture. These 

 marks are useful not only to members of the Survey who desire to relocate positions, but 

 also to private surveyors who desire to connect their survey with one of these control points. 



Every survey must have a beginning, 

 and the beginning point of the great na- 

 tional triangulation survey is a point in 

 Kansas formerly known as the United 

 States Datum, but now as the North 

 American Datum. All surveyors' lines 

 in the United States eventually will lead 

 from here, as all roads led to Rome in a 

 bygone civilization. Canada and Mexico 

 have adopted the North American Datum, 

 so that practically the whole of North 

 America which has been connected trigo- 

 nometrically is hitched to this point. 



So carefully have all the different sur- 

 veys been made that they may all be put 

 together and the lines fit perfectly. Any 

 one familiar with surveys made on each 

 side of a river, and how hard it is to get 

 them to fit together, in ordinary practice, 

 can understand what accuracy has been 

 required to bring all the different sur- 

 veys of North America into such har- 

 mony that from every point already fixed 

 a skilled man from the Coast and Geo- 

 detic Survey could run his line right back 

 to the North American Datum, in Kansas. 



Of late years many cities have found 

 it necessary to order a triangulation of 

 their territory, in order to connect their 

 surveys with those of the Coast and 

 Geodetic Survey. There was much con- 

 fusion encountered in the offices of city 



surveyors in trying to connect their 

 layouts of streets, lots, parks, and other 

 works and areas with the big federal sur- 

 vey. These triangulations cover such 

 cities as New York, St. I^ouis, and Bos- 

 ton, and are now in progress in San 

 Francisco and Cincinnati. Maryland has 

 had all her oyster beds located by the 

 United States triangulation surveys and 

 their positions fixed with reference to the 

 United States Datum and the stars. 



DETERMINING THE EARTH'S SHAPE 



The work of the Survey in measuring 

 with great accuracy the surface of the 

 United States has not only had its practi- 

 cal uses, but its scientific uses as well. 

 The diameter of the earth is the primary 

 yardstick by which all celestial distances 

 are measured, and the exact length of 

 this astronomical yardstick cannot be 

 known until the shape of the earth is 

 determined with great accuracy. So im- 

 portant is this work that 21 nations are 

 linked together in an international agree- 

 ment for the support of the International 

 Geodetic Association and the promotion 

 of its work of earth-measurement. A 

 central bureau is maintained at Telegra- 

 phenberg, Prussia, and triennial confer- 

 ences of the science of geodesy are held. 



The work of the Coast and Geodetic 



668 



