Why were Tuvalu’s islands placed reliably on European maps only in the 1820s?

Although Europeans first encountered the islands of what is now Tuvalu during the 16th-century voyages of the Spanish explorer Álvaro de Mendaña de Neira, they were not reliably placed on European charts until the 1820s.12

This delay occurred because early European exploration in the Pacific was often transitory, and the islands were not consistently visited or documented by cartographers until the 19th century. The reliable mapping of the islands was specifically facilitated by the increased presence of whalers and traders in the region during the 1820s, whose frequent voyages provided the necessary data to accurately record the locations of the island group on European maps.3412