The Triangulation of North America

FOR many years the triangulation of North America existed as a number of detached pieces. Most of the arcs have now been combined into a single system. In this respect the triangulation of North America has followed the usual method of development incident to this class of work. When the systematic survey of the coasts of the United States was begun about a hundred years ago, triangulation was extended from each of a number of astronomic stations located in strategic points. Whenever two or more of these separate coastal arcs were joined they were harmonized by a local adjust? ment, but much remained to be done before a single system could be formed for the whole country. The United States Coast Survey, when first organized, had for its sole function the location of the coast-line and the execution of surveys necessary for the production of hydrographic charts. To serve as control for these coastal surveys, an arc of first-order triangulation was extended from Maine to Delaware Bay along the coast and thence south-westward along the foot? hills of the Appalachian Mountains to Mobile, Alabama; and on the Pacific coast an arc of first-order triangulation was extended from the 40th parallel southward to the Mexican Boundary. It was early realized that the coasts of the Atlantic and Pacific oceans and the Gulf of Mexico should be joined by a transcontinental arc of triangulation in order to secure their proper coordination. This was accomplished by the execution of an arc of first-order triangulation along the 39th parallel of latitude extending from the Atlantic to the Pacific. Field work on this arc was begun in 1871 and finished in 1896. While this work was in progress the interior triangulation was found to be of such great value for many purposes other than the coordination of the coastal charts that the Coast Survey was charged by the federal government with the execution of geodetic surveys throughout the interior of the country, and the name of the bureau was changed in 1873 t0 Coast and Geodetic Survey. In making the interior surveys the Coast and Geodetic Survey was assisted in the north-eastern part of the country by an organization called the Lake Survey, a branch of the Corps of Engineers of the United States Army, which was directed by Congress to chart the Great Lakes along the boundary between Canada and the United States. That organization executed a system of first-order triangulation extending from the eastern end of Lake Ontario to the western extremity of Lake Superior, and also ran an arc overland from the western end of Lake Erie to the southern extremity of Lake Michigan, and another from Lake Michigan through Eastern Illinois to a connection with the transcontinental arc along the 39th parallel. The next great arc of first-order triangulation was started in the late '90's by the Coast and Geodetic Survey and completed in 1916. It extends along the 98th meridian from the mouth of the Rio Grande, latitude 260, to the Canadian Boundary, latitude 490. As explained later, this arc has also been carried through Mexico by the geodetic organization of that country, and in

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