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Speed limit sign in the Republic of Ireland, using "km/h.". The SI representations, classified as symbols, are "km/h", "km h −1" and "km·h −1".Several other abbreviations of "kilometres per hour" have been used since the term was introduced and many are still in use today; for example, dictionaries list "kph", [3] [4] [5] "kmph" and "km/hr" [6] as English abbreviations.
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. Recent British Admiralty charts have a latitude scale down the middle to make this even easier. [10]
Some airspeed indicators in aircraft prior to the mid-1970s indicate in miles per hour plus knots (1 knot = 1.15 mph) or kilometers per hour (1 knot = 1.85 km/h). A primary flight display with the indicated airspeed (IAS) displayed in the form of a vertical "tape" on the left.
Airspeed is commonly given in knots (kn). Since 2010, the International Civil Aviation Organization (ICAO) recommends using kilometers per hour (km/h) for airspeed (and meters per second for wind speed on runways), but allows using the de facto standard of knots, and has no set date on when to stop.
The UK national speed limit of 60 miles per hour (97 km/h) for ordinary vehicles applies beyond this sign. In some countries, derestriction signs are used to mark where a speed zone ends. The speed limit beyond the sign is the prevailing limit for the general area; for example, the sign might be used to show the end of an urban area.
The fastest possible speed at which energy or information can travel, according to special relativity, is the speed of light in vacuum c = 299 792 458 metres per second (approximately 1 079 000 000 km/h or 671 000 000 mph). Matter cannot quite reach the speed of light, as this would require an infinite amount of energy.
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Here the speed is displayed both in knots (kn) and miles per hour (mph). The true airspeed (TAS; also KTAS, for knots true airspeed) of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft.