Why if I go to Greenwich observatory in Goggle Earth does it not show exact Lon 0 on the meridian line?

Answer 1

Back in the 19th century, when the Greenwich observatory was the most important observatory in England, there were many 'datums' in use in different parts of the world. A datum is just a theoretical model of the world. Most people preferred to use models that made their mean sea level the world's mean sea level, etc. Each also tended to make the 'prime meridian' match their location.

When there was a shift to using a common world-wide model, everyone's local coordinates (as they used to measure them in their own local datum) shifted, some by several hundred meters.

The most common datum in the US (and most common in aviation world-wide since they especially need a standardized system) is the World Geodetic Society's 1984 model (the 1996 version of it). Typically, the WGS revised their models as better experimental data was obtained - that meant coordinates could shift again. The two digits at the end represented the year the model was created. With the popularity of GPS navigation, the WGS-84 became such a common datum used on so many maps that it would been very expensive to implement a new model to reflect better knowledge about the shape of the Earth - everyone would have to buy new maps. The solution was to still use the old model, but update the constants slightly, hence you have a 1996 version of WGS-84.

The most accurate model is the International Terrestrial Reference Frame. It's something people can use if they absolutely need a more accurate model, but then their model isn't compatible with most of the world's maps.

There's also a few quirks in using a standardized model. For instance, the highest point on the island of Diego Garcia in the Indian Ocean is listed at about 55 meters BELOW mean sea level. Obviously, a standardized model sacrifices a few realities, but at least it provides a standardized reference for navigation systems and works as long as you understand it is just a reference system (in other words, who cares how far above or below 'sea level' you live - just as long as you don't get wet).