Magnetic chart errors and secular changes in the Indian Ocean

Abstract

In view of the exceptionally large errors in the most recent charts which are being disclosed in the Indian Ocean on the present cruise of the Carnegie, it was thought desirable to make early communication. Tables I and II give the values of the magnetic declination (“variation of the compass”) with the geographic positions, as determined on the Carnegie from Cape Town to Colombo, Ceylon, thence to Mauritius, April to August, 1911; a future communication will give the results from Mauritius northward to the Arabian Sear thence to Colombo and Batavia. While the present figures are not final, they are not likely to be changed by more than one‐tenth degree.The tables also give the corresponding values as scaled from the British Admiralty chart of 1907 (B); the German chart of 1910 (G); and the United States Hydrographic Office chart of 1910 (U); these chart values have been referred approximately to the time of our observations by applying the secular changes as indicated on the respective charts. The final three columns give the differences between the Carnegie values (C) and those of the various charts; the plus sign (+) means that the charts give west declinations too large, the minus sign (−) on the other hand signifying that the chart values are too low by the tabular amount.