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George Huxtable wrote:
> Why should the sea surface not be an equipotential near the shore? Is he 
> talking about gravitational effects from the adjacent land-mass? Or about 
> the effects of tidal inertia? Or what? Are these effects, whatever they are 
> due to, likely to be big enough to affect dip observations?
>   
Although I am not sure I assume it was the proper relation between the 
near and off shore tides what he needed for his tests. When you take the 
observations it is the tide at the horizon that matters. Just as the 
near shore sea surface the sea surface at the horizon goes up and down 
with the tides (with exception of the amphidromic points of course). Now 
when you want to check the influence of dip from the shore using the 
tide to simulate a change in observer height, you need to know exactly 
how the tide behaves at the horizon, while you measure it near the shore 
(that is where your tide station usually will be). Just imagine that an 
amphidromic point is near the spot where you measure the horizon using 
your sextant or theodolite, there will be no relation between the 
measured tide near shore and the observed dip (and you might think that 
the observer's height does not influence dip). Even worse; if the 
amphidromic point is between the observer and the horizon, the horizon 
might rise, while the near shore tide falls.

Further more the shape of the geoid might not be smooth between the 
observer and the horizon, which will have its effects as well, 
especially on theodolites as they use the local vertical as reference. 
So when you want to compare sextant observations with the ones taken 
with a theodolite you need to know (or determine) the astro-geodetic 
deflection for your location as well. The deflection can be as large as 
one arc minute.

Nicol�s


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