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Herbert,

You wondered '... where to find a location that is "a good one for
making refraction dependent measurements"?'

With my comment I meant dependent on refraction near the horizon,
including dip. At higher altitudes refractions are fairly accurate and
constant values. It appears that the refraction near the horizon did
not receive the attention to be systematically analysed and that the
observed variations have been accepted as changing "weather" (as Frank
called it). The generally accepted values for refraction agree with
values resulting from calculations based on simplified, ideal model
atmospheres. I have however not come across studies showing the
effects from non-standard atmospheric features on these accepted
values. It is therefore up to the observer to judge whether the
measurement was made in a situation corresponding to the model
atmosphere or not, i.e. whether for the given situation the proposed
refraction value should be trusted or not. There are certain
situations where one knows from experience that the situation is
considerably different (e.g. Santa Ana or F�hn), then there are the
situations where we can have reasons to *think* that the refraction
value would be wrong (like noticing a local heat source), and finally
there are situations where one only could notice a non-standard
condition from additional observations of environmental parameters.
One such environmental parameter is the shape of the rising/setting
sun. If the shape does not show an abnormal deformation while
rising/setting one can reasonably assume to have "normal" conditions.

Marcel


On Sun, Apr 21, 2013 at 2:14 AM, Herbert Prinz
 wrote:
> ________________________________
>
> Marcel,
>
> I just wonder where to find a location that is "a good one for making
> refraction dependent measurements"? I guess you may handpick a few selected
> places in the open ocean that are far from land and free of currents.
> Unfortunately, these are the places where the navigator has the least
> interest in exactitude.
>
> Most places where we navigate feature notorious warm or cold surface
> currents with side arms, changing eddies, counter eddies and what not. While
> they are often obvious to the naked eye, the corresponding abnormalities of
> refraction go undetected. Frank's photos show that they must be there.
>
> Herbert Prinz
>
>
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