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The Accuracy and Use of Sextants and Watches in Rupert's Land in the 1790s

Summary The purpose of this paper is to draw attention to the sextants and watches employed at the end of the eighteenth century in surveying Rupert's Land, the vast territory of the Hudson's Bay Company (HBC), and to discuss their usage and accuracy. The results provide information on the standards of manufacture at the time, on the reliability of the contemporary latitude and longitude of various fur-trading posts, and on the careers of the surveyors themselves. The sextant readings are shown to have a standard deviation of about 20 arcseconds when used in a vertical position to find altitudes of the Sun and stars, and about 40 arcseconds otherwise, for finding lunar distances. Because of the use of artificial horizons, these conclusions imply that, at a 95% (2σ) confidence level, a single determination of latitude is reliable to ±20 arcseconds (600 m on the Earth's surface), of longitude ±20 arcminutes (approximately 60 km in Rupert's Land), and a single determination of local apparent time is reliable to ±3 s. The watches, it appears, could run at a steady rate with errors of less than a minute for a period of a week or two.