In view of the appeal for observations that has been issued by the Institute to assist in the determination of the accuracy of astronomical observations at sea, it may be of interest to navigators to recount some of the experiences of a hydrographic surveyor. Large regular surveys usually depend on geographical positions which are obtained on shore with accurate instruments. In cases where surveyors fix these positions with astrolabes, or occasionally theodolites, three observers will probably have taken observations of some thirty-two stars each. In the analysis, intercepts of the order of 4–6” are obtained, and the final position is probably accurate to about 1”. Some hydrographic surveying, such as a one-day search for a reported shoal out of sight of land, various minelaying and minesweeping operations out at sea, and the determination of oceanographical stations on long passages, however, demand ordinary sights taken in the normal way at sea. The value of the hydrographic operation may depend on the accuracy of these sights, and therefore the report on the operation generally attempts some assessment of their accuracy. Considerable care is taken to achieve accuracy and often the ship is stopped and the sights referred by range and bearing to a moored beacon, even in depths of 500 fathoms. Methods of determining the accuracy obtained may be by taut-wire runs from known points of land or taut-wire distances interconnecting several beacons. Failing, or in addition to, these methods, two to four observers' results may be compared, or Sun and star sights can be referred to the same beacon; even short tautwire or patent-log runs to well-defined and previously well-fixed fathom lines have been used.