2172 HYDROGRAPHIC MANUAL PagE 68 



and Delaware. In 1901, the adopted datum was officially named the "United States 

 standard datum," and in 1913, with its adoption by both the Dominion of Canada and 

 the Republic of Mexico, its designation was changed to the "North American datum." 

 In 1927 a unified adjustment was begun of all the first-order triangulation in the 

 country in order that certain unavoidable discrepancies that had accumulated in the 

 various arcs of triangulation would be systematically distributed throughout the whole 

 network. In this adjustment the latitude and longitude of station MEADES 

 RANCH were held fixed, but a new value of the azimuth to station WALDO was 

 adopted based on the various Laplace azimuths distributed through the network. 

 To distinguish between values on the North American datum and values in the new 

 adjustment, the term "North American datum of 1927" was adopted. The position 

 of MEADES RANCH on this datum is: 



Latitude 39° 13' 26". 686 



Longitude 98 32 30.506 



Azimuth to WALDO 75 28 09.64 



Computed on Clarke's spheroid of 1866. 



A station is said to be on the North American datum of 1927 when it is rigidly 

 adjusted to the scheme of the readjusted triangulation. 



All hydrographic surveys in waters bordering continental LTnited States are now 

 made on the North American datum of 1927. However, independent geodetic datums 

 exist in the territories and insular possessions of the United States and the surveys 

 therein are, with minor exceptions, based on the datums listed below: 



Hawaiian Islands Old Hawaiian datum 



Philippine Islands Luzon datum 



Puerto Rico and vicinity Puerto Rico datum 



Canal Zone Panama^ — Col6n datum 



Alaska : 



Southeast Alaska North American datum of 1927 



West of Glacier Bay to Mt. St. Elias Southeast Alaska datum 



The 141st meridian Yukon datum 



Cape St. Elias to Wide Bay on the Alaska Peninsula (including Eodiak 1.). Valdez datum 



Latitude 57i,4°, Alaska Peninsula to Atka I., Aleutian Islands Unalaska datum 



And various independent astronomic datums. 



To differentiate between an independent datum and the North American datum of 

 1927: An independent datum is established primarily for horizontal control over a 

 limited area with reference to an assumed starting point, whose position on the spheroid 

 of reference by latitude and longitude and whose azimuth to an adjacent station have 

 been determined astronomically, but which has not been connected to the standard 

 datum of the continental area. 



The hydrographic surveys bordering those limited areas are naturally controlled 

 by their respective datums. However, all independent datums on the North American 

 Continent will probably be connected to the North American datum of 1927 at some 

 future date, thereby making it possible to bring them into the general adjustment, 



2172. Sounding Datums 



For use on the nautical charts of the tidal waters of the United States and its 

 insular possessions and territories, planes of reference or sounding datums have been 

 chosen in accordance with existing conditions. The sounding datum for depths is 

 mean low water (MLW) for the Atlantic Ocean and Gulf of Mexico; and mean lower 



