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» Online Classroom   »   » Public Discussion of Cel Nav   » Dip short calculations

Author Topic: Dip short calculations
Mike Buchko

 - posted April 06, 2004 05:02 PM      Profile for Mike Buchko           Edit/Delete Post 
Thought I'd mention a quirk in the starpilot software. As is expected (for small distances) the shorter the distance the larger the dip correction. However at some point depending on Height of Eye, the dip short equation breaks down and as your dip short distance increases you actually end up with larger dip corrections as distance increases. At the point that this occurs you need to revert to your standard dip calculation .97sqrt(He). Unfortunately the software doesnot have an automatic check for this so you must determine the point yourself. Specifically the eq Ds = 0.4156d + 1.856 h/d
falls apart at large d because h/d becomes zero and you end up with a linear function for Ds which is soley dependant on d and this is obviously incorrect. So make sure if you use the dipshort part of the routine you are aware that there is a danger of overestimating the dip correction with it if you blindly toss a dip distance that is too large into the box.

David Burch

 - posted April 07, 2004 11:16 AM      Profile for David Burch           Edit/Delete Post 
Thanks. This is a valid point. I have run across it myself. This will be addressd in the next build of the program.

My procedure is to take sqrt(HE) and if the distance to the shoreline below the body is farther than that i do not use dip short. (HE in feet, distance in nmi.)

More specifically, you are usually doing this on land from a known position as part of a practice exercise. So the procedure is to do the sight reduction, get Zn, lay it out on the chart and then measure the distance to the shoreline, and then make your choice as to whether or not dip short is required. Once this is done and you know HE and distance accurately, the dip short procedure works remarkably well.

I will add a note in the StarPilot tech support.

From: Starpath, Seattle, WA
Mike Buchko

 - posted April 09, 2004 12:08 PM      Profile for Mike Buchko           Edit/Delete Post 
Thanks David , That is what I've been doing and I was actually surprised by how well it works.
Our class did some moon sights at the pier at White Rock last week and they estimated the dipshort distance to be 1 nM (more or less by just guessing) but when I checked it by plotting the Zn it ranged between 1.5 nM and 2.0 nM depending on the time of the sight. And with He = 19 feet the difference between 1 nM and 2 nM is about 4'.

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