All you ever wanted to know about geodesy

If you've ever wondered what the difference between a coordinate system and a datum is, or what the difference between OSGB36 and WGS84 is, or what is the relationship between the ellipsoid and the geoid, or even what is the difference between a TRS and a TRF, then I recommend the excellent Ordnance Survey paper A guide to coordinate systems in Great Britain. It's an extremely well written explanation from first principles of the underpinnings of GPS - the coordinate and reference systems which are used to locate a point on the planet. Although the title suggests it is UK-centric, much of it is in fact globally applicable.

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