31-Oct-2004 -- Visiting the Greenwich Royal Observatory last October, I enjoyed the exhibitions in the museum. It is free of entry, so on a Sunday afternoon it is quite busy. At the meridian line I took a picture of myself as everybody usually tends to do. This picture was taken towards northern direction with Greenwich Town and the Millennium Dome in the background. If you turn around 180º at the meridian line, you will face the Observatory's Meridian Building and looking at an atomic clock.
Above the clock is a sign 'PRIME MERIDIAN OF THE WORLD, CENTRE OF THE TRANSIT CIRCLE, LATTITUDE 51° 28' 38", LONGITUDE 0° 00' 00". This is the exact location which was agreed to in 1884 in Washington to define the meridian. More precisely, it was agreed that the meridian line would be defined through the cross-hairs in the Airy Transit Circle eyepiece. That way, a landmark that indicates 0° longitude and the start of the Universal day was set in the real world.
I was able to get direct access to the exact location where Sir George Biddell Airy's Transit Circle was located in 1884 (that is where the demarcation of the Meridian intersects with the Meridian Building). I was also able to use my GPS and get a position accuracy of 6m. The coordinates displayed read 51° 28' 40.4" N, 0° 00' 05.4" W! Something seemed to be inconsistent here. Since I knew that one arc second is roughly 30m at the earth surface, I wasn't standing at the meridian at all (according to the GPS reading).
I entered Airy's magic coordinates (51° 28' 38" N, 0° 00' 00" EW) as a waypoint and the GPS receiver software suggested going another 127 m in South-Eastern direction in order to reach the real point! Well, I did, which meant that I had to leave the observatory through the gate and walk a little bit along the parkway, where I was able to 'zero down' the coordinates.
Following my addiction to document degree confluences, I took a picture of the GPS display and the four cardinal direction points. Of course I didn't miss the required picture of the 'confluence'.
It is located on the middle of a wide walkway with conker trees along both sides, which were in beautiful autumn foliage at that time of the year. Perhaps, this was the actual origin of our world-wide grid and nobody was aware of it? At least this was the case according to the GPS receiver based on the WGS84 geodetic datum. Strictly speaking the previous visit to 51° 28' 38" N, 0° 00' 00" EW was an incomplete one according to the 100m-rule which is set up by the confluence.org team. The rules clearly rule out any other systems than WGS84.
After I had documented the point, I headed back home with my bicycle and started brooding about the gap between the two systems. First, I found the following information on Ordnance Survey GPS webpages (chapter 4.1):
...all lines of latitude and longitude in the WGS84 datum, are not stationary with respect to any particular country. Due to tectonic plate motion, different parts of the world move relative to each other with velocities of the order of ten centimetres per year. The International Reference Meridian and Pole and, hence, the WGS84 datum, are stationary with respect to the average of all these motions. But this means they are in motion relative to any particular region or country. In Great Britain all WGS84 latitudes and longitudes are changing at a constant rate of about 2.5 centimetres per year in a north-easterly direction....
Well, no. Over the last two decades since 1984, the total movement of Great Britian would be about 50 centimetres and therefore not be an explanation for the 127m gap that I had discovered.
So, why didn't it fit? To answer this question, we need to have a look at the definition of both systems:
1.The meridian was agreed upon in October 1884. At the behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., USA for the International Meridian Conference. At the conference the following important principles were established: […The meridian passing through the principal transit instrument at the Observatory at Greenwich was to be the "initial meridian". That all longitudes would be calculated both east and west from this meridian up to 180°. The universal day would be a mean solar day, beginning at the mean midnight at Greenwich and counted on a 24-hour clock…]
But what does mean, "the cross-hairs in the eyepiece of the Transit Circle precisely defined Longitude 0° for the world."? Let's have a look at how such an astronomical instrument works:
A transit circle is used to make two measurements - the time at which a star crosses the meridian, as it travels from East to West across the sky, and the angle of the star above the horizon.
Horizon! That's the key issue here. The instrument is levelled with a surveyor's bubble. The air seeks the highest point in the bubble level and with adjusting it between some markers the instrument is parallel to the horizon. Since the horizon is perpendicular to the plum line it is obvious, that the horizon depends on the direction of gravity. In other words: the gravity at Airy's Transit Circle location was decisive for Airy's magic coordinates. But we all know, how irregular the earth gravity field (geoid) is. Since the distribution of mass in the inner earth is irregular due to different densities, so is the gravity field.
2. Now let's have a look at the definition of the WGS84 system:
WGS84 is an earth-fixed Cartesian coordinate system with:
Its "z-axis" is aligned parallel to the direction of the Conventional Terrestrial Pole (CTP) for polar motion, as originally defined by the Bureau International de l'Heure (BIH), and since 1989 by the International Earth Rotation Service (IERS).
Its "x-axis" is the intersection of the WGS84 Reference Meridian Plane and the plane of the CTP Equator (the Reference Meridian being parallel to the Zero Meridian defined by BIH/IERS).
Its "y-axis" completes a right-handed, earth-centred, earth-fixed (ECEF) orthogonal coordinate system, measured in the plane of the CTP Equator, 90 east of the x-axis.
We can see, that WGS84 is a system based on geometry, not on gravity. Interesting for us are the definition of the z- and the x-axis. In order to define the z-axis, they had watched the rotation of the earth at epoch 1984.0 (that is, midnight on New Year's Eve 1983), and then took the result of their observations to define the z-axes and therewith the latitudes.
The WGS84 x-axis points to the Zero Meridian of the BIH. And how was that Zero Meridian defined?
The WGS84 reference meridian passes through Greenwich and is specifically defined by the BIH zero meridian at epoch 1984.0
We see, the longitudes are defined in a similar fashion, based on rotation observations in the year 1983. Obviously, this meridian did not exactly match Airy's astronomical observation.
If I transform the GPS readings that I had at Airy's Meridian point (51° 28' 40.4" N, 0° 00' 05.4") into conform coordinates, I get a northing of 5705252m and easting of 500104m. This means, that the deviation along the meridian is 74m and in eastern direction is 74m. Again this makes a diagonal deviation of 127m between the WGS84 and the Airy's point.
As a conclusion I can say that both systems are defined on entirely different physical approaches: Firstly, the direction of the plumb line was determined in an astrogeodetic way using Airy’s telescope. Secondly, a GPS receiver was used for measurement of pure geometric ellipsoidal coordinates. Comparing both will clearly produce differences. In Geodesy they are called vertical deflections and quantified in angular degrees. In Greenwich I measured them as ξ=2.4" and η=5.4".