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Just a bit more about Andrew Denham's question on magnetic variation.

It relates to a GPS receiver, the French-made MLR 24, which Andrew and I
both possess. Sorry to bring in this forbidden topic of GPS once again, but
it has raised serious questions about the meaning of magnetic variation.

On my own receiver, I have until now preferred to leave the "compass var."
at its preset value of zero, preferring to work always in true, so have
never noticed the error that Andrew relates.

But now I can confirm exactly what he says. With "compass var." set to 0,
and a waypoint with latitude exactly 1 deg further North than my present
position, and identical longitude, then a course to that waypoint of 360
deg is shown, and a distance of 60 miles, exactly as one might expect.

However, with "compass var." set to 10W, that same waypoint is shown at a
course of 350 deg (and again 60 miles). The course should of course be 10
deg. The compass variation has been applied in the wrong sense.

There's no need to consider + and - conventions, or how the French might
approach the matter. It's simply WRONG. There's a serious bug in the
software. When you input a Westerly deviation, it treats it as Easterly,
and vice versa.

Andrew's understanding is quite correct. He has put his finger on a real
problem. The response from the distributors is no more than bluster. The
manual is silent on the matter. I will try to discover more.

George Huxtable.

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george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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