![]() Recent British Admiralty charts have a latitude scale down the middle to make this even easier.Greta Sans is designed as a continuous optical size system. Since the length of a nautical mile, for practical purposes, is equivalent to about a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart. A single graphic scale, of the sort on many maps, would therefore be useless on such a chart. On a chart of the North Atlantic, the scale varies by a factor of two from Florida to Greenland. Standard nautical charts are on the Mercator projection and the horizontal (East–West) scale varies with latitude. As a result, nautical miles and knots are convenient units to use when navigating an aircraft or ship. Although the unit knot does not fit within the SI system, its retention for nautical and aviation use is important because the length of a nautical mile, upon which the knot is based, is closely related to the longitude/latitude geographic coordinate system. The difference from the modern definition is less than 0.02%. This method gives a value for the knot of 20.25 in/s, or 1.85166 km/h. The knot count would be reported and used in the sailing master's dead reckoning and navigation. Knots tied at a distance of 47 feet 3 inches from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation. The chip log was cast over the stern of the moving vessel and the line allowed to pay out. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. Until the mid-19th century, vessel speed at sea was measured using a chip log. ![]()
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