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Subject: Re: Sextant Positions versus Map Datums
From: John Kabel (jjkabel@XXX.XXX)
Date: Fri Jan 18 2002 - 08:57:51 EST
[This was part of an answer to Trevor Kenchington yesterday. I'm
sorry if the message is a repeat. I never got a bounce from the
list, so I am not sure it got through. My ISP is having BIG problems
right now.]
I have looked through both my Nautical Almanac and the 1992 edition
of the Explanatory Supplement to the Astronomical Almanac. In
neither is there any mention of adjusting for datum after a sight
reduction.
The NA is referenced to the plane of the equator and the longitude
line through Greenwich, England. All "predictions" for star and
planet positions are referenced to these entities. All astronomical
effects of inconsistency of earth rotation, polar wobble and the
effects of solar system bodies on each other's orbits are taken into
account and "buried" in the tables. The system appears to be Earth-
centred by definition.
I would suggest that in the past, locations of foreign datums were
determined by celestial means, relative to Greenwich and the equator,
probably with instruments with proper levelling attachments, so they
would be vertical to the geoid.
So, all the charts in a particular area will be
already be historically referenced to the standard on which the
Almanacs are based. If sailing near Japan, for example, one would
already be doing the DR on charts in the Tokyo datum, and working up
plotting sheets based on those charts. One would not be using a
chart based on NAD-27, unless you were really missing the point of
safe navigation.
Page 21 of my 1995 Bowditch also discusses Datum Shifts, especially
the problem of moving between charts with different datums. "If any
position is replotted on a chart of another datum using only latitude
and longitude for locating that position, the newly plotted position
will not match with respect to other charted features. This datum
shift may be avoided by replotting using bearings and ranges to
common points. If datum shift conversion notes for the applicable
datums are given on the charts, positions defined by latitude and
longitude may be replotted after applying the noted correction."
Bowditch also gives four suggestions for minimizing errors caused by
different datums on the same page.
Again, I suggest that an attempt to place all one's sights in a
global reference is meaningless, since you practically have to adjust
onto new charts as you move around the world. Sights are relative to
DRs in the local datum, not absolute, at least not at our level.
John Kabel
London, Ontario
43d 01.177'N , 81d 12.089'W, give or take 10 m.
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