In fact the Earth isn't a sphere, it's an oblate spheroid, flattened at the poles and with bumps on it. These bumps are not the mountains and oceans, but irregularities which depart from the regular shape of the Earth. They occur over areas large enough for the mapmakers to take them into account when producing maps of their country. Because the bumps effecting each country is slightly different, each national map making organisation has used complicated formulae which best represent their part of the world, so that other
map makers know what is going on, each formula is designated a code, known as a map datum. This is printed on the chart. For example, in the United Kingdom, the map datum is referred to as OSGB36, which stands for Ordnance Survey of Great Britain, 1936, the year in which it was devised. In the United States of America, it's NAD27, but this is then split into versions for different areas and in Australia it's Aus Geo 66.
Having different map data mattered not one jot. Although the same latitude and longitude may have given positions several hundred metres from their proper position, the 'accuracy' of position fixing was such that these errors were undetectable. Not so now, where even amateur yachtsmen can fix their position to within 15 metres most of the time.
This difficulty was realised when GPS was being designed and a new international datum was produced which, although very complex, allowed for the true shape of the Earth everywhere. This datum is known as WGS84 - World Geodetic System 1984.
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