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On the answer to Calculating Latitude/Longitude X miles from point? what is the tc value?

lat = asin( sin(lat1)*cos(d)+cos(lat1)*sin(d)*cos(tc) )
dlon = atan2( sin(tc)*sin(d)*cos(lat1),cos(d)-sin(lat1)*sin(lat) )
lon = mod( lon1-dlon+pi,2*pi )-pi
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tc stands for True Course.

The term True Course is used in navigation to express the bearing in which the ship is traveling. It is used to differentiate that measured from the geographic north with respect to those measured from the magnetic north.

In your formula, it represents the bearing of the distance (arc lenght) d at the starting point (the angle at the vertex of the spherical triangle formed by the starting point, the end point and a pole). Because d is the distance measured over a great circle, and over a great circle the True Course varies depending on the latitude, so that sin(tc)*cos(lat)=constant.

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