I was recently at Greenwich and standing at the zero longitude meridian line. I was surprised to see that this did not coincide with a tile boundary (I was using zoom level 14 tiles in Veloviewer). So just wondering where the downloadable tiles ‘start’ from?
It’s a bit involved. The maths is here but the tiles are probably a bit beside the point.
Angles from Greenwich were originally measured using gravity to determine which way was ‘up’, but the Earth is lumpy and not a uniform density. The ‘local up’ actually misses the centre of the Earth slightly.
GPS and most other GNSS (at least consumer grade stuff) uses WGS84, as does OSM. This attempts to be centred on the centre of mass of the Earth but AFAIK doesn’t give particular weight to Greenwich when setting the 0 point.
This video attempts to give a partial explanation, but I’m sure it omits loads as the correct response when GIS people start talking seriously about coordinate systems is normally to smile and back away slowly.
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