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Thank you George for the background info.I didn't know that at one time
people were trying to determine longitude useing mag. var.
After reading your post I have a few questions/comments about what you
wrote.Perhaps I missunderstand what you were saying.Bear with me as I try to
understand.
In my training and experiance one uses amplitudes to find the compass
error.By adding or subtracting the variation one finds the compass deviation
at that heading and time.With out knowing the deviation how can one know the
variation useing amplitudes?In your explaination is the sight taker useing a
hand held bearing compass or a sight vane on the ship's compass or
repeater?There could be a differance because if one is useing a hand held
compass to observe the amplitude one doesn't take deviation into account as
one does on the gyro or ship's compass's sight vanes.
Let's tackle amplitudes 1st.I know of 2 ways one can find an amplitude of a
body.By observing the body's amplitude on the visible or celestial horizon
with a compass and useing the amplitude tables or by calculateing the body's
amplitude on the visible or celestial horizon.
In either precedure above the resulting amplitude is compared to the
observed amplitude thus finding the compass error.
Keiran stated he has a way to find local variation useing " a
sextant,compass and amplitude".How can this be done?Am I missing something
or are my thoughts to confined ie. ship board application only?

   

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