NavList:

>Jim Wilson asked -
>"I understand that the purpose of the double altitude sight is to get time
>of meridian passage. Is there another use?"

To which Geoffrey Kolbe replies: ......

"What I meant was the same accuracy as the commonly available GPS
receiver, where the circle of uncertainty in position has a radius
about 5 metres."
Geoffrey Kolbe

=========

Dear Geoffrey,

Having just read this, I am dubious of the claim for accuracy of a circle of position of 5 Meres with anything other than observatory standards in equipment; and if no radio time signals available - over a long period of time to establish the clock accuracy, and only with sophisticated and accurately divided circles on the Zenith / altitude measuring instrument.

In "ordinary" use by mariners and a sextant, I doubt this would be possible even with land based observations and an artificial horizon; with measurements no better than say 0.2 minute of arc accuracy; and worse, time only possible to within a few seconds of absolute with a chronometer. Even today with at least guaranteed accurate time signals; to achieve that circle of position would take some doing with a sextant. Even with a theodolite reading to say 20 seconds of arc I would expect it to be difficult exercise.

I am open to correction on this of course so would like to know others' opinions.

What would be the required accuracy of observation in time and altitude to achieve 5 Metre radius of position? The experts here will post this quickly enough no doubt.

Douglas Denny.
Chichester. England.

==============================
Original Posting:

>Jim Wilson asked -
>
>"I understand that the purpose of the double altitude sight is to get time
>of meridian passage. Is there another use?"
>
>and Geoffrey Kolbe replied-
>
>".... In fact, before the advent of GPS technology, it was probably the most
>accurate way of deriving longitude available - giving very similar
>precision.

George Huxtable said

>When Geoffrey says "giving very similar precision", then I ask- similar to
>what?

I am guilty of an incorrect use of the term, "precision". I should
have said, "accuracy"

What I meant was the same accuracy as the commonly available GPS
receiver, where the circle of uncertainty in position has a radius
about 5 metres.

Geoffrey Kolbe

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