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Good morning,

Some words to introduce myself:
I'm new to this forum, and this is my first post.
I live in Reunion Island, in the Indian Ocean (E55?30' S21?00), where I
work as engineer in surveying.
Being at first interested by land navigation, I have been a GPS addict
since may be 1995. Nevertheless, this let me frustrated to not to
understand the traditional way of positionning, so I learn Celestial one
year ago. I practice it at first with a theodolite, and then bought
recently a Cassens & Plath CPSailing sextant (it is in fact a disguised
Astra IIIB).

Then come back to the topic:

Fried Squash a ?crit:
>
> Due to the oblateness of Earth, the plumb line differs
> from the actual center point of Earth by a
> navigationally significant amount, up to 12 minutes of
> arc.
> But this fact does not effect the practice of
> celestial navigation because the lat-lon grid itself
> and the Almanac's GP data are based on the plumb line.

It takes me a while to understand this, but I agree.

> The local plumb line may differ by additional amounts
> as well, called deflections, due to surface anomalies,
> but not by a navigationally significant amount.

Near mountains or steep islands, the deflection can be high enough to
affect geodesical measurements. In the place where I live the deflection
is 0.7', which is considered one of the highest in the world. As I
understand that a bubble sextant is not that accurate, this should not
affect navigation. Nevertheless, If you need accuracy, you have to take
it into account when you use a theodolite.

Best regards

Pierre Brial

   

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