| Contents Page | Next --> |
GNSS systems report positions using all sorts of conventions, so we have to look at the different ways that a position can be represented.
This guide from the UK's mapping authority The Ordnance Survey (OS) gives an excellent introduction.
The OS also provides this web page which converts a position expressed in one coordinate system into all the others. (Beware: the accuracy is limited!)
For use offline, the OS offers this spreadsheet tool which does similar conversions, (maybe more accurately, I haven't checked yet).
What follows is my take on the subject.
According to current theories the Earth and the Moon were formed about 4.5 billion years ago when two planets collided. The heat generated by the impact caused the two bodies to melt and then merge. Some of the material was ejected and coalesced to form the Moon.
The resulting Earth has a solid core at the centre, mainly iron, very hot but under huge pressure, which stops it from melting. Above that is more iron, under less pressure and liquid. It's the rotation of that which generates the Earth's magnetic field. Further out from the centre is the mantle, essentially a hot gooey mess of rock, flowing like a very viscous liquid. The solid surface crust floats on top of the mantle.
The material in one of the original two planets was a little denser than the material in the other and there are still huge lumps of this denser material slowly moving around, deep below the surface.
As a result the centre of gravity of the Earth is not at the same position as the geometric centre and both move around. The axis of spin passes through the centre of gravity, causing the poles to move with it.
The crust is broken into large plates which move around slowly, pushed and pulled by convection currents in the mantle.
On the surface, the tides move huge volumes of water backwards and forwards during the day. Even a nearby mountain can have enough material in it to deflect local gravity slightly. These and other factors cause local gravity to be uneven so that it doesn't always pull directly towards the centre of mass of the Earth.
To sum up, nothing on the Earth is fixed. There's no solid ground. There's a thin shell, relatively solid, around a huge boiling maelstrom of molten fluid. Convection currents deep inside stir up the material, causing the centre of gravity, the axis of spin, the magnetic poles and the continents on the surface to move around.
The Earth's crust is composed of tectonic plates which move around the surface and change shape slowly due to continental drift . The plates move at different rates and in different directions. The part of the European plate that I live on moves at about 2.5 cm per year heading approximately north-east. The USA moves in a different direction and the two plates move apart at about 3cm per year. Iceland sits at the point where the two plates part which causes all sorts of geophysical effects, some quite violent.
This slow movement didn't used to matter so much to surveyors because their equipment was not sensitive enough to notice it. Now it can be detected using equipment costing only a few hundred dollars and so it has to be taken into account.
The Earth is held together by its own gravity and is almost, but not quite, a sphere. It spins on an axis, which stretches it very slightly at the Equator and shrinks it very slightly at the poles, making it approximately an ellipsoid.
As explained here this was predicted by Newton's Theory of Gravity and established in the 1730's by two French surveying expeditions, one to Lapland and one to Ecuador. They each measured the distance covered on land of one degree of latitude and showed that the result in Lapland, close to the North Pole, was not the same as the result in Ecuador, close to the Equator. This BBC programme describes the expedition to Ecuador.
Newton's theory assumed that the density of the Earth was uniform so it predicted that the Equator is circular, meaning that the Earth is a Spheroid. According to this page "A Spheroid is simply a type of Ellipsoid which is as wide as it is long (i.e. circular around the middle and close in shape to that of a sphere). All other Ellipsoids are longer than they are wide (i.e. have an oval shape between ends and in the middle, like a football)."
In 1830, the British Astronomer Royal George Airy defined the Airy Spheroid which was a good approximation to the shape of the Earth, given the data available at the time. In 1936 the UK Ordnance Survey defined the OSGB36 coordinate system based on Airy's spheroid. They still use OSGB36 for their paper maps. More on that here .
As surveying techniques improved it became clear that the Equator is not quite a circle so a spheroid is not a good enough fit. Geographers now use a more general ellipsoid that's close on average to the Earth's real shape. It's called the Ellipsoid (capital E).
The Ellipsoid is not always a convenient tool for making maps, and some cartographers define other ellipsoids that better approximate the shape of the locality that they are interested in.
Just to confuse the matter even more, the term "spheroid" is also sometimes used interchangeably with "ellipsoid" in cartography, even though one is a special case of the other.
More on that whole subject here .
As already mentioned, gravity doesn't always point at the centre of mass of the Earth. There's another solid shape called The Geoid (which just means "Earth-shaped" in Latin).
According to Wikipedia: "The Geoid is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended through the continents."
You can find more about The Geoid here .
The Geoid is approximately at mean sea level at the points where the land meets the sea, so it's a good replacement for the rather vague concept of mean sea level.
As with ellipsoids, mapmakers often define their own geoids which more closely match their local conditions.
To sum up, ellipsoids (sometimes called spheroids) are regular solids that approximate to the shape of the Earth and geoids are irregular solids that also approximate to the shape of the Earth. The Ellipsoid and the Geoid (with capitals) are designed to work globally but local ellipsoids and geoids can approximate the local landscape more conveniently for local maps.
| Contents Page | Next --> |
Copyright © Merrow Internet Services Ltd 2024