15 



files an entire new projection was applied to each. As more surveys were 

 made and more adjustments required it was impractical to apply full pro- 

 jections to the sheets, therefore the practice in recent times has been to 

 add only an occasional grid intersection to show the correction to be 

 applied. 



Two spheroids of reference, Bsssel's and Clarke 1 s, have been used by 

 the U. S. Coast and Geodetic Srrvey. However, various datums have been 

 used on each. The old Besse! 's Datum was in use from 183-4 to 1 844. and 

 the Bessel's spheroid of 184.1 was used until 1880 when Clarke's spheroid 

 of 1866 was adopted. The datums used on Clarke's spheroid have been: 



Clarke's Datum 1880 - 1901 



U. S. Standard Datum 1901 - 1915 



North American Datum 1915 - 1927 



North American 1927 Datum 1927 to date 



The North American Datum and the U. S„ Standard are the same,, the 

 only difference being the name. The term U. S. Standard was changed 

 when the datums of Canada and Mexico were tied in to the U. S. network 

 making it, as the name implies, a North American Datum. 



In 1927 an adjustment of all first-order triangulation in the United 

 States was begun -based on station "Meades Ranch" in Kansas, which was 

 selected because of its location near the center of area of the United 

 States on two major arcs of triangulation extending across the country at 

 right angles to each other, and because of the great amount of recomputation 

 that could be avoided since no change wpuld result in the network that had 

 been extended to that point from the New England States and other coastal 

 states southward to North Carolina. When the adjustment to this base has 

 been made the stations are on the North American 1927 Datum. It is to this 

 datum that the historic surveys are adjusted for superposing for study by 

 beach erosion engineers. 



A truly satisfactory method of showing shore line and offshore depth 

 changes has not been developed. Many attempts have been made but none 

 satisfy all conditions. The favored method used by the Beach Erosion 

 Board is to adjust the graticules of each of the surveys to be used to 

 the North American 1927 Datum. Tnis gives a projection common to all 

 sheets. A composite tracing is made of the shore lines and selected off- 

 shore depth curves using symbols to differentiate between the various 

 periods. The lateral shift of the shore line and depth curves can then 

 be determined. 



It has been mentioned that the adjustment of some of the old surveys 

 present a problem. These cases are rare. When such a sheet is found it 

 should not be abandoned until a check has been made in the old records of 

 the Geodesy Section and in old publications of the Coast and Geodetic 

 Survey which give the position of "lost" stations. 



