What is the real radius of earth?

  • Google says 6378.1 km
  • Wiki says 6371 km
  • And there are references for 6378.7 km

I'm using maximum radius 6378.7 km in distance calculation in spherical law of cosines formula. What would be the impact of using maximum radius rather than average radius which is 6371 km. When does using one over the other make sense?

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  • PolyGeo, searching to see if the question had been asked would have been useful, but otoh, so would having the original question show up under the Related header. Just a point, not a complaint. – recurvata Aug 30 '14 at 16:34
  • @recurvata I'm not certain that I understand your comment but I think making this one a duplicate should cause the original to show up as related next time someone starts to write one similar to this. – PolyGeo Aug 30 '14 at 21:05

In short: It really depends on what spheroid body you use to approximate earth, because in reality earth is not a sphere, not even an ellipsoid, but a complex irregular shape that cannot be described mathematically. That said, all those radius values may be correct. The radius (or semi and major axis of an ellipsoid) is usually part of something called a spatial datum, which is a set of parameters that define your system for describing, or better, approximating earth in a mathematical way. Which datum you choose depends on what you are trying to achieve, what is the size of your area of interest (one city, country, maybe entire world), and where it is located.

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