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Earth radius (denoted as R 🜨 or R E) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid (an oblate ellipsoid), the radius ranges from a maximum (equatorial radius, denoted a) of nearly 6,378 km (3,963 mi) to a minimum (polar radius, denoted b) of nearly 6,357 km (3,950 mi).
In any ellipsoid, the length of a degree of longitude at the equator is exactly 60 geographical miles. The Earth's radius at the equator in the GRS80 ellipsoid is 6,378,137.0000 m, [3] which makes the geographical mile 1,855.3248 m. The rounding of the Earth's radius to metres in GRS80 has an effect of 0.0001 m.
The "old English mile" of the medieval and early modern periods varied but seems to have measured about 1.3 international miles (2.1 km). [17] [18] The old English mile varied over time and location within England. [18] The old English mile has also been defined as 79,200 or 79,320 inches (1.25 or 1.2519 statute miles). [19]
One link is a hundredth part of a chain, which is 7.92 inches (20.1168 cm). [7] The surveyor's chain first appears in an illustration in a Dutch map of 1607, [8] and in an English book for surveyors of 1610. [9] In 1593 the English mile was redefined by a statute of Queen Elizabeth I as 5,280 feet, to tie in with agricultural practice.
The "mile line" of the 1652 Cromwellian settlement, prohibiting Catholics settling within a mile of the Connacht coast or River Shannon, was an English statute mile. [ 6 ] [ n 1 ] A 1653 survey of lands of the Countess of Ormonde in County Kilkenny used statute acres, whereas the 1654–5 Civil Survey and 1655–6 Down Survey used plantation ...
The rod, perch, or pole (sometimes also lug) is a surveyor's tool [1] and unit of length of various historical definitions. In British imperial and US customary units, it is defined as 16 + 1 ⁄ 2 feet, equal to exactly 1 ⁄ 320 of a mile, or 5 + 1 ⁄ 2 yards (a quarter of a surveyor's chain), and is exactly 5.0292 meters.
A graphical or bar scale. A map would also usually give its scale numerically ("1:50,000", for instance, means that one cm on the map represents 50,000cm of real space, which is 500 meters) A bar scale with the nominal scale expressed as "1:600 000", meaning 1 cm on the map corresponds to 600,000 cm=6 km on the ground.
As one degree is 1 / 360 of a circle, one minute of arc is 1 / 21600 of a circle – such that the polar circumference of the Earth would be exactly 21,600 miles. Gunter used Snellius's circumference to define a nautical mile as 6,080 feet, the length of one minute of arc at 48 degrees latitude. [24]