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The true meridian is the chord that goes from one pole to the other, passing through the observer, and is contrasted with the magnetic meridian, which goes through the magnetic poles and the observer. The true meridian can be found by careful astronomical observations, and the magnetic meridian is simply parallel to the compass needle.
True north is the direction along a meridian towards the geographic North Pole. Somewhat more formally, Bowditch defines variation as "the angle between the magnetic and geographic meridians at any place, expressed in degrees and minutes east or west to indicate the direction of magnetic north from true north. The angle between magnetic and ...
Burt's instrument allows surveyors to determine the true north direction in reference to the Sun rather than being influenced by the Earth's magnetic field. [18] It is made of brass and therefore has no magnetic influence on a compass needle, [18] as it was originally a small attachment to a standard surveyor's common compass. [25]
Maps issued by the United Kingdom Ordnance Survey contain a diagram showing the difference between true north, grid north, and magnetic north at a point on the sheet; the edges of the map are likely to follow grid directions rather than true, and the map will thus be truly rectangular/square. [citation needed]
With one exception (magnetic declination), they all depend on a common principle, which is to determine the time for an event or measurement and to compare it with the time at a different location. Longitude, being up to 180° east or west of a prime meridian, is mathematically related to time differences up to 12 hours by a factor of 15. Thus ...
The WGS 84 meridian of zero longitude is the IERS Reference Meridian, [8] 5.3 arc seconds or 102 metres (335 ft) east of the Greenwich meridian at the latitude of the Royal Observatory. [ 9 ] [ 10 ] (This is related to the fact that the local gravity field at Greenwich does not point exactly through the Earth's center of mass, but rather ...
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The alignment of the spin axis is permitted to rotate in azimuth by only the small amount required during operation. An initial approximate estimate of the meridian is needed. This might be determined with a magnetic compass, from an existing survey network or by the use of the gyro-theodolite in an extended tracking mode.