RULES AND PRACTICES 9 



In 1899 an adjustment of the first-order triang;ulation in the United States was begun, 

 based on station Meades Ranch in Kansas. This station was selected because of its location 

 near the center of the area of the United States and because it was on two major arcs of tri- 

 angulation extending across the country at right angles to each other. Another advantage of 

 adopting this origin was that no change was required in the network that had been extended 

 to that point from the New England States and other coastal states southward to North 

 Carolina. In 1927 a new unified adjustment was made, also based on AMEADES RANCH, read- 

 justing positions in the east as well as the west. See the Hydrographic Manual for additional 

 details (Publication 20-2, pages 235-236). 



The North American Datum of 1927 is the standard datum for nautical charts. With few 

 exceptions, all charts of the continental United States and Alaska are on the North American 

 Datum of 1927. Therefore, "N.A. Datum of 1927" need not be shown on these charts. How- 

 ever, the older surveys are not on this datum and care must be exercised to see that they are 

 properly corrected before application to the charts. 



The Old Hawaiian Datum is the geographic datum authorized for the Hawaiian charts. 



In the Caribbean area, charts are on the Puerto Rico Datum. 



The polyconic projection is now used for all field sheets. The fact that the projection 

 lines are not straight or parallel should not be overlooked when correcting the older surveys 

 to N.A. Datum of 1927. 



Differences between the old and new (1927) geographic coordinates of the triangulation 

 stations shown on the sheets should be used when making corrections. An up-to-date file of 

 the geographic coordinates of triangulation stations used on survey sheets is maintained in 

 the Nautical Data Branch. 



ISOGONIC LINES 



Isogonic lines shall be shown on those charts on which a variation of 1° will, in general, 

 result in a distance between the lines of less than 12 inches. 



COMPASS ROSES (U) 



Compass roses are placed on the charts to facilitate plotting bearings and laying out 

 courses. The magnetic variation is changing constantly due to the fluctuations of the earth's 

 magnetic fields. Because of this fluctuation, compass roses are brought up to date on the 

 charts whenever it is necessary to make a new printing plate. The roses now being used on 

 the charts of this Bureau were constructed on glass by the Bureau of Standards and are 

 considered very accurate. They consist of an outer circle in degrees, with zero at true north, 

 and an inner circle in points and degrees with the arrow indicating magnetic north. See 

 Chart No. 1 for example. 



