NavList:

   

Reply

> Okay, a sextant position is based on the almanac which is based

>Okay, a sextant position is based on the almanac which is >based on...ergh...

All the astronomical models like VSOP87 are accurate to far more than we need 
for celestial navigation and do it over a timespan of 6000 years.  In order 
to plot star position to the nearest arcsecond, they need to take the slight 
bulge of the earth into account.  It would be a second order effect, as the 
parallax of even different sides of earth orbit is infinitesimal for a star, 
so we are only talking about a very slight change in angle of the surface.  
It is a small enough percentage that for accuracy to the arc-minute it may 
not be needed for stars, and certainly it isn't in the sight reduction 
procedure.  This leads me to believe it is irrelevant for navigational 
accuracy.

However, if you look in the nautical almanac, you will see in the appendix on 
methods of sight reduction, the correction for a lunar observation due to 
parallax includes a small correction (on the order of 0.1 to 0.2 minutes due 
to parallax depending on latitude.  I hope I am not off on that but I don't 
have the almanac handy at the moment, so that number is from memory.  So, for 
lunar measurements, the oblateness of the earth is relevant, and for anything 
else it is not.

>If NAD27 and WGS84 disagree by a mile, then wouldn't
Do they in fact disagree by a mile?  That would surprise me.

> on...ergh...How exactly does that match up to a map datum? Should
> (*) OK, the various datums use slightly different >ellipsoids, but I
>believe that the horizontal difference is not significant >for
>this discussion. The difference is more evident in the  -- >Peter

I think Peter hits it on the head.

   

Reply