# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Measuring (and Calculating) Dip
Date: 2013 Mar 1, 09:52 -0500

Hi Marcel

Thank you for the critique.  It is most welcome.

Perhaps it is a translation error.  In re-reading Friesenleben's paper, he indicates to take the difference in degrees C between the surface temperature and the temperature of the air at the height of the eye.

That's not clear to me.
Is it
Air-Water?
Or
Water-Air?

When he says to take the temperature at the height of the eye, does that mean at the location of the eye?  Or can I substitute the buoy location because the buoy's thermometer is roughly at the height of the eye?

My Height of Eye is indeed precisely known.  I directly measure the height of eye at the time of measurement of dip.  I am standing on dry land.  Variation will be caused only by the tides.

I have been in contact with Dr. Young, who has graciously provided me with his paper.  There are no direct equations contained therein.  He states that we should use Friesenleben (check!) and to use Hasse for wave heights.

On wave heights, this appears to be a geometric argument.  That is, instead of the horizon being mean sea level, it is rather the crests of the waves.  I need to give this more thought but preliminary geometry for an observer on land gives wave corrections on the order of 1 arc minute, with the correction getting smaller as the height of eye increases.

What to be made of the observer bobbing up and down as well as the waves?  The navigator is cautioned to take his observation when at the crest of the wave.  Does this not negate the geometric wave argument?

Fortunately, the NDBC buoys give wave information in exquisite detail.  Not only do we have the wind wave amplitude and period, swell amplitude and period, as well as the significant wave amplitude, we also are provided the power spectral density of the waves.  As a surfer, I am all over this data and know it well.

Regards

On Mar 1, 2013 7:31 AM, "Marcel Tschudin" <marcel.e.tschudin@gmail.com> wrote:

Brad, regarding your calculation of the dip with formulae mentioned in Freiesleben (1950): Note that the temperature difference can be positive or negative. I understand that you should use in these equations for the temperature difference T of air at eye level minus T of sea, but you better verify it again. You do not have T at eye level. I guess that the mean of the air temperatures measured by the two buoys may possibly be an approximation of it. This shows that the temperature differences, and, depending on the wave heights, also the height of eye will likely contain a considerable part of uncertainty. As a consequence of this one may expect heavily scattering differences when comparing calculated with measured dip values.

Again, sorry for having given before a misleading information regarding Sunset Science II. This publication explains properties of the dip but does not contain an explicit dip formula as you were looking for and now found in the publication from Freiesleben.

Marcel

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