The Eastern Oblique Arc
The Eastern Oblique Arc is a triangulation of the eastern United States extending from Calais, Maine to New Orleans, Louisiana. The triangulation served several purposes, perhaps foremost was to tie together the multitude of coastal and harbor surveys into a uniform network, thereby providing a datum on which to base further surveys. Once the triangulation was complete, it served a secondary purpose as a basis for computing a spheroid to best fit the continental United States.
The work was carried out in several projects; it extended throughout the better part of the 19th century, beginning in 1833 with the first triangulations from Long Island, New York, to the Chesapeake Bay, and ending with the work from Atlanta, Georgia, to Mobile, Alabama, which was completed in 1898. The final report on the arc was published 1902.
As the work proceeded, adjustments were published periodically and formed the basis of the early datums of the United States, beginning in 1851 with the first publication of geographic data for use by the public, and continuing with the publication of the initial New England Datum in 1871 and its subsequent adjustments. In 1901, the CGS incorporated data from the newly completed Transcontinental Triangulation along with the earlier work done in the East to publish the U.S. Standard Datum.
As a basis for computing the shape of the earth, an oblique arc (one extending diagonally to the meridians), required accurate measures of longitude to be useful. In 1846, telegraphic methods began to be employed for accurate measures of differential longitude, and accurate absolute longitude was determined using the transatlantic telegraph in 1866 and 1867. An oblique arc also required more difficult calculations. The analysis was finally completed in 1902 and published in a 394 page volume, Special Publication No. 7 of the U.S. Coast and Geodetic survey.
Timeline of the work
- 1833 - 1844 The triangulation from Long Island, New York, to the upper Chesapeake Bay.
- 1846 - 1874 Coastal triangulations from Mobile, Alabama to New Orleans, Louisiana.
- 1859 The triangulation from Rhode Island to Calais, Maine.
- 1865 - 1871 Work in Connecticut to connect the triangulations, and extension down the Chesapeake Bay to across the Potomac at Washington, D.C.
- 1873 - 1877 Extension from Washington, D.C. to Atlanta, Georgia.
- 1885 - 1898 From Atlanta, Georgia to Mobile, Alabama.
The triangulation incorporated six base line measurements.
- 1834 The Fire Island Base Line (14,058.9709 +/- 0.0585 meters, Long Island, New York).
- 1844 The Kent Island Base Line (8,687.5446 +/- 0.0680 meters, Chesapeake Bay, Maryland).
- 1844 The Massachusetts Base Line (17,326.3763 +/- 0.0358 meters).
- 1847 The Dauphin Island Base Line (10,661.8376 +/- 0.0260 meters, Mobile Bay, Alabama).
- 1857 The Epping Base Line (8,715.9422 +/- 0.0158 meters, Epping, Maine).
- 1872-3 The Atlanta Base Line (9,338.4778 +/- 0.0222 meters, Atlanta, Georgia).