Latitude and longitude reference coordinates.?

When you use lat. and long. options on your GPS, it is usually with reference to WGS84. Working in North Africa we used to be given coordinates for oil wells etc. in Clarke1880 format; as the difference could be as much as 2km, we had to get them to redo it in WGS84.

What causes the difference, and why is there one at all? If I worked out my latitude and longitude with a sextant and chrononometer, what system would I be using? What system do Ordnance Survey maps use, and what about Google Earth?Latitude and longitude reference coordinates.?
The whole thing gets a bit complicated, but here is an outline of the problem.



The earth can be regarded as a sphere and it is possible to base a system of navigation on this. There is a problem, the earth is not a sphere but is flattened at the poles because the rotation of the earth results in bulging at the equator. So a more accurate method is to regard the surface as an elipsoid. There are parameters which describe the flattening at the poles and are used to generate the elipsoid. It is also possible to generate a geoid. This can be done by assuming an extension of mean sea level over an area covered by land.



There is also the problem of a datum. That is basically a known point from which we measure the relative possitions of all other points.



Now for a bit of history related to the USA but applicable to other countries with modifications for any locally derived datum.



A standard, the North American Datum 1927 (NAD27), used the Clarke ellipsoid derived by Clarke in 1886 and a base line starting at Meades Ranch in Kansas. The origin point is located on the surface of the earth.



When satellite data became available, it became apparent that the ellipsoid was incorrect and for this and other reasons some 2,000, 000 reference points spread across North America were also inaccurate.



A new standard, the North American Datum 1983 (NAD83), is referenced to the WGS84 ellipsoid. Coordinates are related to the centre of mass of the earth, not to a surface location. This has resulted in a more accurate system but has "moved" the 2,000,000 reference points by as much as 300mtrs.



So the difference is because of a better deffinition of the elipsoid and a better reference point.



Your reference to Africa indicates that they had previously been using the Clarke ellipsoid and that the oil companies were using the WGS84 ellipsoid.



Satellite navigation systems generally use the 1G geoid as a reference. This is an imaginary surface on which the acceleration due to the earth's gravity is 1G. This is usefull becasue it takes in to account local variation and is, of course, the surface that is followed by the satellites orbit. GPS systems then translate the gathered information to WGS84 coordinates. If you look at all the information that is available from the GPS, one of the things will be your altitude above the 1G geoid.



Using star shots, the simplest thing to do is to determine lattitude from observation of the pole star (when in the northern hemisphere). Longitude is determined from sun observations and an accurate clock. These observations are obviously modified with changes in altitude and need some correction.



Ordnance survey maps use a UTM projection. This provides for accurate projection of the earth's surface onto a flat sheet of paper. The accuracy is limited to a small area which is why the world is split up into UTM zones. An artificial grid is used which is not alligned to lat/lon. You will see on the paper maps that they show a direction for grid north, true north and magnetic north. The map will also state which ellipsoid and which datum was used to produce the map. Errors exist, as with all maps, and increase towards the edge of the map. This is why you cannot just take two paper maps and glue them together and expect everthing to match up.



I do not know what Google earth does but I would think that they reference it to WGS84.



I hope this helps.
Units of Measurements are always dependent upon country or an organisation. Just like In UK, we use term Miles while in India, we use trm Kilometre for measuring city distances. To resolve your issue, why not refer to SI units of measurements (uniformed standard of units maintained by France)?Latitude and longitude reference coordinates.?
OS AND GPS NOW USE THE SATELLITE NAVIGATION FIXED POINT WHICH SHOULD ELIMINATE THE MARGIN OF ERROR A SEXTANT WOULD GIVE.Latitude and longitude reference coordinates.?
The answer is more complex than that, but basically, most of the difference comes from the fact that WGS84 and Clarke1880 describe earth as slightly different ellipsoids.