In primary triangulation the lengths of the sides of the triangles are so great that they can not be used as a basis for a detached survey of a small tract within one of the triangles. For this reason the primary triangles I, II, III, etc., are subdivided by a system of secondary triangula tion, and these secondary triangles are again divided into tertiary triangles. The specific methods of triangulation used by the United States Government Surveys are described under Topographical Surveys, in the article SURVEYING. It may be mentioned further here that the three great surveying organizations of the Govern ment, the Coast and Geodetic Survey, the Engi neer Corps of the Army and its allied commis sions, and the Geological Survey, are all engaged in extending systems of primary triangulation upon which to base their detached surveys. The Coast and Geodetic Survey has already carried a chain of triangulation around the entire coast line of the United States as a base for its charts. It has also extended a system of triangulation of the greatest geodetic refinement into several coastal States and a great belt of transcontinental triangulation connecting the Atlantic and Pa cific coasts. The Engineer Corps has extended a system of triangulation around the entire shore line of the Great Lakes and throughout the lengths of the Missouri and Mississippi rivers.
The Geological Survey has extended the work of the Coast Survey and the Engineer Corps into the interior of a considerable portion of the country. Several of the States, and particularly New York and Massachusetts, have triangulated much of the area within their boundaries.
The important general problems of geodesy re lating to the earth's figure are also receiving much attention. The civilized governments of the world have formed an International Geodetic Association which meets annually. Delegates attend from the geodetic departments of the sev eral nations, and their joint deliberations make possible a concerted attack upon large problems, and give the benefits of cooperation. At the same time duplication of work can be avoided.
Some important works on geodesy are: Clarke, Geodesy (London, 1888) ; Hagford, Geodetic As tronomy (New York, 1898) ; Merriman, Geodesy (New York, 1899) ; Wright, Adjustment of Ob servations (New York, 1884) ; Jordan, Handbuch der Vermessungskunde (Stuttgart, 1857) ; Hel mert, Theorien der Geoclitsie (Leipzig, 1880-84) ; United States Coast and Geodetic Survey annual publications; similar publications are issued by most European governments.