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

Base Lines

The triangulation incorporated six base line measurements.

More Details (North to South)

EasternObliqueArc (last edited 2009-03-21 22:34:16 by JimIrwin)