The cache is not at the listed coordinates – at least, not in the normal sense.
Since the dawn of humanity, people have speculated over the shape of the world we live on and its place in the universe.
Hindu Mythology describes the world as a disc, supported by four elephants (or snakes –they’re easily confused) on the back of a giant turtle swimming through space.
In the 16th Century, Nicolaus Copernicus proposed an alternative scheme, explaining the absence of observable turtles and serpentine elephants by turning the Earth into the ball in a gigantic game of swingball (if you picture an invisible, 93 million mile long string attached to a stake that looks like a huge, white-hot ball of fire that explodes at the end of the game, you will appreciate how easy it is to take an analogy too far).
Mapmakers have long been attracted to this idea, not least because:
- Claiming the World isn’t flat when a customer complains his map won’t lay flat on the table might buy you enough time to take his money and disappear.
- It explains why ships seem to sink as they sail into the distance, then return a few weeks later - without all that yucky stuff with Bill Nighy dressed up as a squid
- It allows for a system of coordinates (Lat, Long) based on angles round the globe that can be measured by looking at where the Sun and stars appear in the sky.
It started to get messy when people discovered that the planet is 25 or so miles smaller across the poles than it is across the equator. Scientists have spent much of the last 400 years bickering, in the name of a science called “Geodesy”, about which elephant had sat on the North Pole, how far it kicked the Greenwich Observatory and whose fault it all was - while pretending to explain the whole thing away with “centripetal” force.
For the mapmaker, this is a headache of (literally) literally global proportions. Before you can say where a hill is and how far it sticks out above the surface, you need to be able to say where this “surface” is. Although the earth is only about a third of a percent flatter than a sphere, 25 miles of flattening is hard to ignore when you’re trying to fix positions to within a metre.
For the last couple of hundred years, most serious mapmakers have built their maps around a shape called an ellipsoid. It’s still not perfect, but you can define an ellipsoid that matches sea level to within 200m worldwide. The ellipsoid used by GPS – it’s called GRS80 – is one of these global best fit ellipsoids.
If you’re prepared to focus on just one region, you can choose other ellipsoids (different in terms of size, eccentricity, orientation or location) which fit that part of the world far better (but are more or less useless everywhere else). Ordnance Survey maps use one (the 1830 Airy Spheroid) which is about 1km smaller than GRS80, a bit less flattened and pushed away from the centre of gravity of the Earth. The Airy Spheroid matches mean sea level all around the UK really closely, but if you tried to use it globally, you’d end up mapping Bondi Beach at about 3000 Ft above sea level.
Having chosen an ellipsoid, you need one more thing to set up your coordinate system – a prime meridian (a line drawn across the surface passing through some arbitrarily selected point – using your own capital city is generally good for a knighthood – that you choose to call Zero longitude). For the Ordnance Survey, this was originally a line drawn through the Royal Observatory in Greenwich. In 1936, it was shifted a little in a fudge to average out the surveying errors when the original network of trig points had been laid out a century earlier. The combination of the 1936 prime meridian with the 1830 Airy Spheroid gives rise to a coordinate system called OSGB36.
The WGS84 coordinates used by GPS use the GRS80 ellipsoid, combined with a new prime meridian (the IERS Reference Meridian) which misses the Greenwich Observatory by about 100m. Why the difference? Because the people who ran the first satellite navigation system (TRANSIT) were based at Johns Hopkins University in the USA and really didn’t see why the Lat/Long of their office had to change when they moved to a global ellipsoid.
If you look in the margins of an OS map, alongside the familiar eastings and northings you’ll find Latitude and Longitude scales. These are drawn using the OSGB36 scheme – and don’t match the WGS84 position you’d get from your GPS. The difference varies depending on where you are in the UK, but is often more than a hundred metres.
The posted Lat/Long coordinates describe the cache location during August 2015 using the OSGB36 datum on the Airey Spheroid. (The cache, along with the whole of the UK, is drifting north-east at about 2.5 cm a year – so in 70 years or so, I’ll need to update the coordinates).
The cache is a 2litre cliplok box. When placed, it contained three TBs and the usual £1000 note for FTF.
Congratulations to Seherezade for FTF.