It is based upon a triangulation by the U.S. Coast and Geodetic Survey.
Duffield and published in the Report of the U.S. Coast and Geodetic Survey for 1895-1896 as Appendix 12, pp. 395-7 22.
Besides these there is a vast amount of material in the collections of the Bureau of Education, the Bureau of Ethnology, the Smithsonian Institution, the National Museum, the House of Representatives, the Patent Office, the Department of Agriculture, the Botanic Gardens, the Bureau of Fisheries, the Naval Observatory, the Geological Survey and the Coast and Geodetic Survey.
From 1817 he directed the triangulation of Holstein, to which a few years later was added a complete geodetic survey of Denmark (finished after his death).
Having become assistant to Carlos Guillelmo Moesta (1825-1884), director of the observatory at Santiago, in 1859, he was associated with the Chilean geodetic survey in 1864.
