When visitors to the Royal Observatory, Greenwich
stand astride the Meridian, they are often perplexed to discover that their
GPS does not give their longitude as zero. Likewise, users of Google Earth
are sometimes surprised to see that the Meridian as marked, appears to pass
around 100 m to the east of where they expected.
The explanation for these apparent anomalies is rooted in the history of
longitude determinations, the assumed shape of the Earth and the way in
which maps have been historically constructed.
When the Royal Observatory was founded back in 1675, it was widely believed
that the Earth was spherical. This notion was challenged by Newton with the
publication of the third volume of his Principia in 1687 in which he
hypothesized that the Earth was an oblate spheroid, also known as an
ellipsoid, the shape generated by spinning an ellipse on its minor axis. He
estimated the equatorial diameter would differ from the polar by about 1
part in 230. The parameters of the ellipsoid have since been refined, but
the ellipsoid is not a perfect fit either.
From the earliest of times, it was a priority for astronomers to get an
accurate determination of the difference in longitude between the
observatory at Greenwich and ones elsewhere. In the case of the Paris
Observatory, there had been at least 18 different determinations by the
1920s. The first attempt to accurately fix the relative longitude of an
American observatory, that of Harvard Observatory in Cambridge,
Massachusetts, took place in the 1840s when nearly four hundred chronometers
were transported back and forth across the Atlantic. From the 1860s onwards,
following the laying of the first transatlantic cable, the time difference,
and hence longitude difference between the two observatories, was determined
with even greater precision by telegraphic means. It was through this single
connection that the longitude (relative to Greenwich) of all other places in
the United States was originally determined.
Most accurate maps show only a small part of the Earth’s surface. Before the
space age, when choosing an ellipsoid to represent the shape of the Earth,
it was the practice to pick one whose surface had a good alignment with
reality over the area of the map. In the UK for example, the maps produced
by the Ordnance Survey were (and still are) based on the ‘Airy Ellipsoid’ –
an ellipsoid defined by the seventh Astronomer Royal George Airy in 1830.
The chosen ellipsoids differed slightly in centre position and orientation
as well as in size and shape. The advent of satellite technology enabled
ellipsoids to be defined for the first time with their centre coincident
with the Earth’s centre of mass.
In the late 1950s (under the auspices of the US Navy), the Applied Physics
Laboratory (APL) of the Johns 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.