Find out how early explorers navigated the seas before they had maps to guide them. People and places - Mapping - Mapping the seas Learning Points Fifteenth-century explorers used dead reckoning to estimate longitude. Explorers in the 18th century used chronometers to calculate their location. A Twig Context Film. Reinforce and extend the learning required by the curriculum. Twig’s context films show abstract concepts in action in the real world.