Discover how techniques for mapping the seas have changed throughout history. People and places -Mapping - Mapping the seas Learning Points The technique of measuring the sea's depth using a weighted line is called depth sounding. Sonar technology allows mapmakers to create increasingly accurate maps of the ocean floor. A Twig Curriculum Film. Delivering key learning points. Get straight to the facts in just three minutes – directly linked to core learning, Twig's curriculum films will fit neatly into your lesson plans.