Chip log
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A chip log is a navigational tool used by mariners to determine speed through the water. A chip log consists of a piece of wood, most often cut in the shape of a quarter circle, or "quadrangle", and a length of rope or line with knots tied every 47 feet and 3 inches (14.4 meters). To determine a ship's speed, the chip log would be placed into the water aft of the ship, line paid out, and the number of knots paid out in 28 seconds counted, giving the speed in knots. A number of considerations had to be taken into account -- the amount of following sea, stretch of the line, and inaccuracies in the measurement of 30 elapsed seconds. Time passage was most often marked with a "30 second glass", a small sand filled glass, which would often run fast or slow based on ambient temperature, humidity, and sea state. Frequent measurements helped in mitigating some of these inaccuracies by averaging out individual errors, and experienced navigators could determine their speed through the water with a fair degree of accuracy. Mechanical chip logs, operating on physical principles in a manner similar to a car's odometer, were eventually developed and replaced the traditional chip log. Today, perhaps the most accurate means of speed measurement comes from Global Positioning Satellite GPS systems.
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