STANDARD GEODETIC DATA. 319 



STANDARD GEODETIC DATA. 



COMMUNICATED BY AUTHORITY OF THE SUPERINTE>JDENT OF THE UNITED STATKS 

 COAST AND GEODETIC SURVEY. 



BY PROF. GEORGE DAVIDSON. 



Read October 18, 1886. 



In the development of the main triangulation of the Pa- 

 cific Coast, it was early discovered that large and irregular 

 deflections of the plumb-line existed at the triangulation 

 stations, whether they were situated on the mountains or in 

 the plains. 



When the main triangulation was undertaken it embraced 

 lines of unusual length, and one part of the scheme was the 

 projection of a network across the continent along the 39th 

 parallel. 



In order to collect standard geodetic data for the compu- 

 tation of the geographical positions on this coast. Assistant 

 Davidson planned at the outset to have the latitude observ- 

 ed at each triangulation point; and he also observed the az- 

 imuth of some one line in the series of directions which were 

 observed from the same station. 



This scheme of trianejulation commenced from an accurate- 

 ly-measured base-line of nearly eleven miles in length sit- 

 uate in the plains of Yolo county, California. From this 

 line it was carried by quadrilaterals to the Coast Eange of 

 mountains, as far west as Mount Tamalpais; and from the 

 line Mount Helena — Mount Diablo it stretched across the 

 great valley of California to the line Mount Lola — Round 

 Top. This scheme of triangulation was named by the Su- 

 perintendent of the United States Coast and Geodetic Sur- 

 vey the " Davidson Quadrilaterals." The observations at 

 all the stations have been shown to be remarkably satisfac- 

 tory, and the discussion has been rigorously carried out in 



