An Admiralty memorandum set out the detailed instructions. The first requirement was to resolve disagreements in the earlier surveys about the longitude of Rio de Janeiro, which was essential as the base point for meridian distances. The accurate marine chronometers needed to determine longitude had only become affordable since 1800; "Beagle" carried 22 chronometers to allow corrections. The ship was to stop at specified points for a four-day rating of the chronometers and to check them by astronomical observations: it was essential to take observations at Porto Praya and Fernando de Noronha to calibrate against the previous surveys of William Fitzwilliam Owen and Henry Foster. It was important to survey the extent of the Abrolhos Archipelago reefs, shown incorrectly in Albin Roussin's survey, then proceed to Rio de Janeiro to decide the exact longitude of Villegagnon Island.