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A Little GPS and Zero Longitude Trivia…
From the Greenwich Meridian Organization…
In the late 1950s (under the auspices of the US Navy), the Applied Physics Laboratory (APL) of the John Hopkins University began the development of what was to become the world’s first operational satellite navigation system. Known as Transit, it worked by making use of the Doppler effect, the same effect that makes a siren carried by a moving vehicle change in pitch as it passes. The surveyed longitude of the Laboratory’s site in Maryland, as measured in the North American Datum (NAD27), became its assumed longitude in the first World Datum, the APL datum. It was this pragmatic adoption of the longitude coordinate on one ellipsoid as the assumed value on another that has caused the apparent shift not only in the position of the Meridian, but also of all other locations.
The size of the shift remained unknown until the summer of 1969, when an opportunity arose to measure it. A satellite receiver was set up on a platform above the roof over the Airy Transit Circle at Greenwich. The results showed that fixes resulting from the use of the satellite navigation system should have their longitude values shifted by 5.64″ if the Greenwich (Geodetic) Meridian was to have its longitude as zero in this system. Although an academic paper on this subject was published in 1971, it appears to have been largely forgotten about until the mid noughties. The offset (since refined) also applies to the WGS84 datum used by current GPS systems. WGS84 was adopted as the global standard for air navigation on 1 January 1998 and soon afterwards by hydrographers for use on electronic and nautical charts.
Until the advent of GPS, local datums were only ever used in a local context. Although usually inappropriate to do so, it is possible with GPS to set a receiver to get a latitude and longitude fix anywhere in the world in any of the different datums. The precise latitude and longitude of a place will vary with the particular coordinate system or datum that is used. Paradoxically, as we have already seen, this also applies to the Airy Transit Circle, whose longitude by definition one might reasonably expect to be zero. The difference between the co-ordinates on different datums also varies from place to place. Most datums agree with each other to within half a kilometre or so. The most commonly used in the UK are OSGB36 & WGS84.
At the time of the International Meridian Conference in 1884, the concepts of continental drift and plate tectonics did not exit. The first evidence of plate movement came in the mid 1950’s as the space age was about to begin. The Earth’s tectonic plates move relative to one another at about the same rate at which human finger nails grow – not much on a day to day basis, but a substantial amount over a period of decades and centuries. With the introduction of satellite technology, came the ability to create a more accurate global datum, and with it the necessity to define a reference meridian that, whilst being derived from the Airy Transit Circle, would also take into account the effects of plate movement and variations in the way that the Earth was spinning. The International Terrestrial Reference Frame (ITRF), which defines the International Meridian and poles, is based on the combination of sets of station coordinates and velocities derived from a variety of different types of observations: Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), and Lunar Laser Ranging (LLR). Data from Global Positioning System (GPS) was introduced in 1991 and from Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) in 1994. The International Reference Meridian and Poles and, hence the WGS84 datum, are stationary with respect to the average motion of the Earth’s crustal plates. As a consequence, all individual locations are in motion relative to them. In the UK WGS84 latitudes and longitudes are changing at about 2.5 cm per year in a north-easterly direction. In 1989, the International Reference Meridian passed an estimated 102.478 m to the east of the Airy Transit Circle at Greenwich.
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4 Replies
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From, Howse, D. (1997). Greenwich time and the longitude. London: Philip Wilson.
Indeed, even the Greenwich meridian itself is not quite what it used to be—defined by ‘the centre of the transit instrument at the Observatory at Greenwich’. Although that instrument still survives in working order, it is no longer in use and now the meridian of origin of the world’s longitude and time is not strictly defined in material form but from a statistical solution resulting from observations of all time-determination stations which the BIPM takes into account when co-ordinating the world’s time signals. Nevertheless, the line in the old observatory’s courtyard today differs no more than a few metres from that imaginary line which is now the Prime Meridian of the world.
From Wikipedia…Apparently, slightly more dubious considering the above.
The origin of the discrepancy dates back to when surveys were being carried out for the Global Positioning System. There is more than one transit instrument at Greenwich, and unfortunately the surveyors picked the wrong one as their baseline.
I assume the Wikipedia version was written by either an engineer or an architect.
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I assume the Wikipedia version was written by either an engineer or an architect.
I assume that you are right….;-)
I hope everyone has a great day; I know I will! -
One reason to miss Bill Strange.
And what of Prof. Cliff Mugnier? Anyone hear from him lately? I believe he’s still active with ASPRS stuff…
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When we were at Greenwich Observatory a few years ago, the Museum staffers indicated that the zero longitude line was about 100 meters east of the Greenwich Meridian, which passes through the Airy Transit.
The line is marked on the ground outside the observatory, and every day hundreds of people straddle it and believe they are standing in both hemispheres.
Original standards for the yard,the foot and other dimensional units are attached to the wall,under the Meridian Clock,along with an original Ordinance Survey Benchmark.
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