Re: Measuring (and Calculating) Dip
From: Bruce J. Pennino
Date: 2013 Mar 19, 18:20 -0400
Frank ,maybe we agree....maybe not.
If I'm on a boat that goes up and down with waves, seems to me I
want to try to measure sight with a sextant at the top of my wave to
the top of the distant wave. But this is probably impractical.
What really happens, I guess, is that I'm somewhere moving up or down
in a wave sighting on a wavy surface/horizon. Hopefully measurement
errors cancel....but probably not. Consequently CN on a moving vessel has a
combination of errors that leads to a band width of location. I believe it is
common wisdom/experience that true location is within a band at
best 2 -3 miles total width.
Now I move to shore with my total station (in the
next couple of days I'll post data showing all of my instrument
measurement and observation errors focusing on near (2.5 miles) and far (12.5
miles) objects). I'm thinking I'll measure 10 discrete vertical
angles always focusing on the highest part of a passing wave, but keeping
the horizontal angle fixed. As the wave goes up and down I'll measure a vertical
angle (dip). If the waves have a long time period, maybe I need to make more
measurements or space them out. When I average all of the data I have a
mean vertical angle. Right? If I measure my height of eye
above the mean waterline on beach , haven't I measured the mean dip
angle with refraction and everything included. Then when you do the sight
reduction, the refraction correction is incorporated into the final
Now if I'm 100,200,300, 500 ft above MSL when I measure a dip angle, waves become less
significant as long as I know my elevation in relation to MSL and I know the
tide level. Of course the tidal variation also becomes less
significant as height of eye is increased.
Based on the errors (scatter in my data with total
station), I want my height of eye to be at least 8 or 10 ft because of
measurement errors that can creep in, not including waves. Incremental dip
angle measurements should be made with at least 60 seconds of difference
in dip. For example, if my height of eye was 8 ft , I should measure about
2.7 minutes of dip ( by standard formula). For next data point (sight), I
want my height of eye to be at least 15 or 17 ft (dip about 4 minutes)
etc. I don't think it is easy to refine dip very
closely/precisely/accurately, even with a "so called" 3 second total
----- Original Message -----
Sent: Tuesday, March 19, 2013 3:26
Subject: [NavList] Re: Measuring (and
Brad Morris, you wrote:
"The wave height correction is by
Oh my, oh my... Brad, this equation is bullshit. I should have been more
blunt in my previous explanations. I really thought that if I spelled it out
to you carefully and slowly, you would see the issues yourself without the
need for blunt language. I guess not.
The correction for wave height is simple: your height of eye should be
measured from the tops of the waves as nearly as possible. If you're on a
vessel on open water, you can look over the side and see those wave tops. If
you're at some protected location, either on shore or in a cove or some other
protected waters, you should count your height of eye from your best estimate
of the wave tops at the horizon. NO OTHER CALCULATION IS REQUIRED.
Here we see another problem with mathematics in celestial navigation.
Sometimes folks get hooked on the "appearance" of accuracy and mathematical
sophistication that comes with a "complicated" equation when the real solution
is both "seat of the pants" practical and mathematically elegant.
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