I decompiled System.Device.Location.GeoCoordinate.GetDistanceTo(...), and found it uses the Haversine formula, which should specify the Earth's radius as one of the steps of calculation.
The radius specified in the decompiled code is 6376500.0
(meters).
In MSDN, the documentation for GetDistanceTo has a remark:
The Haversine formula accounts for the curvature of the earth, but assumes a spherical earth rather than an ellipsoid.
Wikipedia states that
Several different ways of modeling the Earth as a sphere each yield a mean radius of 6,371 kilometers
Also everywhere I search, I find cannot find anyone very few specifying the radius as 6,376.5
km.
So, even though they're pretty close, GetDistanceTo
uses 6,376.5
km, but the widely accepted radius is 6,371
km. Why the difference? Did the creators of GetDistanceTo
figure out a better way to approximate the radius of the Earth?