Among the oldest and most basic tools was the lead line, used since antiquity to measure depth. This measurement could tell sailors how far they were from land. Another low-tech device, the ka-mal, was developed in Asia and the Middle East. It used a piece of wood marked for the position of Polaris at the latitude of various ports. By 1400, more sophisticated versions used a length of knotted string so the navigator could, by placing the string in his mouth, sight the horizon and the height of Polaris to judge latitude.
Timepieces also aided in navigation. In 1400, mariners still used hourglasses. These, in connection with careful observation of shorelines and accurate logbooks, helped navigators estimate location and predict arrival times.