This film by the British Information Service explains how British scientists of the Hydrographic Dept. of the Admiralty chart the seas. Since sands and shoals shift, charts must be updated for accuracy. The echo sounder on shipboard takes depths continuously by sending out sound waves, which are reflected from the sea bottom back to the ship. This is used in conjunction with the sextant's measured angles of landmarks on shore to fix the exact position of the sounding. The triangulation procedure is explained and demonstrated on land, including the use of an astrolabe, chronometer, and chronograph true bearing of the triangulation framework. For triangulation at sea, floating beacons establish baselines, soundings are made, and chart datum of tides' heights for a lunar month is collected. Cartographers then draw charts from the collected information.