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For astronomical calculations (3D) it is required to work from the centre of the body, Usually the mean centre if there is eccentricity
However for navigational purposes the points aimed at need to be surface objects, actual aiming points. Imagine sending a rocket to the moon for a soft landing knowing the distance was 250,000 miles but not knowing the the height of the terrain!
I have never given any thought to these distances before so it is just what I would assume. I may be wrong.
Barry
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