8 NAUTICAL CHART MANUAL 



RULES AND PRACTICES 



The rules and practices set forth herein are those which meet the present conditions and 

 requirements of the Coast and Geodetic Survey. Every effort has been made to bring them 

 into agreement with those of other charting and mapping agencies of the U.S. Government 

 and the International Hydrographic Bureau. 



Although it is desirable that the treatment and appearance of all charts be uniform, the 

 various methods of compilation, drawing and reproduction make this extremely difficult to ac- 

 complish. Changing requirements have a marked effect on the treatment of detail on a chart. 



There are certain rules which can be formulated to govern, to a large degree, the con- 

 struction and revision of a Nautical Chart. The following rules and practices have been 

 standardized as much as possible and when revisions are necessary special instructions will 

 be issued. 



I 



General Practices 



GEOGEAPHIC DATUMS 



Because of redeterminations of the dimensions of the spheroid at various times, and due 

 to the adjustment of additional triangulation nets, several datums have been available for 

 survey operations. 



Two spheroids of reference, Bessel's and Clarke's, have been used by this Bureau in its 

 geodetic triangulation. However, various datums have been used on each. The old Bessel's 

 Datum was used from 1834 to 1844, and Bessel's spheroid of 1841 was used until 1880 when 

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



Clarke's Datum 1880-1901 



U.S. Standard Datum 1901-1915 

 North American Datum 1915-1927 

 North American Datum of 1927 



The North American Datum and the U.S. Standard Datum are the same, the only differ- 

 ence 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 in 1915, making it, as the name implies, a North 

 American Datum. 



