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Some of the off-list discussion of these pesky Meridional Distances has
invoked differing spheroid models of the earth.

My understanding is that once upon a time there were any number of
different spheroid models floating about, leading to confusion, so in an
attempt to establish a constant 'World Geodetic System Spheroid 1984'
was set up and has been generally accepted. Despite this there are still
tables with differing data, even though they may all claim to be derived
according to WGS 1984.

What leads to this, as I understand it, is that the earth is not a
regular shape, not even a regular spheroid ('slightly pear shaped' I've
heard, which is a good image - think of just how irregular the shape of
a pear is), and as if that wasn't enough the two hemispheres are not
even symetrical. So any mathematical model remains just that - a model,
and can only indicate approximate answers for distances across our
watery pear's surface.

However the general assertion remains true, or at least has not been
challenged, that using such models for calculations lead to more
accurate results than assuming distances remain constant despite
latitude change. For me its a valuable bonus that the formulas are
simple and I can (almost!) understand them.

   

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