Page 67 control and signal building 216 



name is more desirable (see 2156). When the natural object is identified by a geo- 

 graphic name, it is almost invariably preferable to use the geographic name or a part of 

 it for the station name. Similarly with structures, it is preferable to select a station 

 name which will identify the structure by description or proper name. 



216. Control Stations of Other Organizations 



Whenever control stations established by other organizations are known to exist 

 in an area to be surveyed, an effort shall be made to connect the new control to a suffi- 

 cient number of the stations of the other organization so that the two surveys may be 

 coordinated. These connections shall be made unless the progress of the party would 

 be materially delayed thereby, or the cost of the project would be materially increased 

 because of the necessity of building and establishing intermediate stations or extending 

 the scheme any considerable distance in order to make the connections. 



Such connections should be made by triangulation or traverse for geodetic coordina- 

 tion. If a geodetic connection is impracticable, a graphic connection should be made 

 by topography. The use of marked stations of other organizations as a part of the 

 control, instead of establishing new stations in the immediate vicinity, is authorized 

 and encouraged, provided they are connected to the federal control network, that their 

 locations are satisfactory for control purposes, and that the station marks are in good 

 condition and conform to the usual standards as to method of establishment, size, 

 strength, and durability. (See 224.) 



217. Datums 

 2171. Geodetic Datums 



A geodetic datum consists of five quantities: the latitude and longitude of an 

 initial point, the azimuth of a line from this point, and two constants necessary to 

 define the terrestrial spheroid. It forms the basis for the computation of horizontal 

 control surveys in which the curvature of the earth is considered. 



The Coast and Geodetic Survey has used two principal spheroids of reference 

 for its geodetic work. Bessel's spheroid of 1841 was used from 1844 to 1880 when it 

 was replaced by Clarke's spheroid of 1866. Clarke's determination is considered 

 the more precise, and later observations have shown that for this part of the globe it 

 represents the true figure of the earth somewhat better than the spheroid of Bessel. 



During the early years of the Bureau's operations many detached triangulation 

 systems existed in the United States, each based on independent astronomic observa- 

 tions within its system. With the completion of the first transcontinental arc of 

 triangulation in 1899, the various detached systems were connected and a coordinated 

 system based on a single geodetic datum was established for the whole country. 



Station MEADES RANCH in central Kansas was selected as the basis for this 

 single geodetic datum because it was near the center of area of the United States and 

 because it was common to two great arcs of triangulation extending across the country, 

 one along the 39th parallel and the other along the 98th meridian. 



The best theoretical position of this station on the adopted spheroid of reference 

 was found to be so near the value obtained by continuous triangulation from the New 

 England States that the latter was adopted to save the labor of a vast amount of 

 recomputation. The adoption of this datum, therefore, did not change the geographic 

 positions of the then completed triangulation in New England and along the Atlantic 

 Coast to North Carolina, nor in the states of New York, Pennsylvania, New Jersey, 



