The station pointer is a triple-armed circular protractor (instruments vary from about 5 to 12 inches in diameter) originally made of brass that is placed over a map or chart to determine a ship’s position (or on land, the user’s location) relative to three distant points that are on the map. Other instruments of navigation include ocean depth gauges or depth sounders which were replaced by echo-sounding and sonar in the 1920s, ships logs, navigational charts, traverse boards or helmsman’s boards which record a ship’s course when it was tracking into the wind and was used with a compass and ship’s log. Traverse boards were in use at least by the sixteenth century. Ancient techniques that were used to estimate a ships speed employed ropes with a log or board tied to the end, hence the origin of ‘log’ or ‘logging’, log-lines or ropes with knots tied at 6-foot intervals (fathoms), were used. Recent ship speedometers from the early 1900s use small foot-long torpedoes with spiral fins attached to a rope with a speed gauge.