United States, Hydrographic Office. Division of Chart Construction, G.W. Littlehales, Hydrographic Engineer in charge ; U.S. Department of the Navy, Bureau of Equiment [...] ; drawn b... U.S. Hydrographic Office.
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2010. National Geophysical Data Center. This map layer shows isoporic lines (lines of equal annual change, or secular variation, in the declination of the Earth's magnetic field), derived... National Atlas of the United States.
2010. National Geophysical Data Center. This map layer shows isogonic lines (lines of constant declination of the Earth's magnetic field), derived from the International Geomagnetic Refer... National Atlas of the United States.
4. Indian Ocean
United States, Hydrographic Office. Depths shown by soundings. Compasses. Magnetic variation curves are for 1940. Includes longitudinal and latitudinal lines, variation and revision c... U.S. Navy Hydrographic Office,.
Great Britain, Admiralty. Drawn for engraving by E.J. Powell, under the direction of Captn. R[ichard] Hoskyn, R.N. Supt. of Charts. Depths shown by soundings; relief by hach... [Admiralty Hydrographic Office],.
6. Indian Ocean
United States, Hydrographic Office. Depths shown by soundings. Compasses. Magnetic variation curves are for 1935. Includes longitudinal and latitudinal lines, variation and revision c... U.S. Navy Hydrographic Office,.
United States, Hydrographic Office. From all available authorities. Depths shown by soundings; relief by hachures and spot heights. Compass rose. Magnetic variation curves are for 193... U.S. Navy Hydrographic Office,.