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» Online Classroom   »   » Public Discussion of Cel Nav   » Labels for IC and related ditties

Author Topic: Labels for IC and related ditties
David Burch

 - posted April 15, 2003 09:17 AM      Profile for David Burch           Edit/Delete Post 
We often preach the values of short cuts or mnemonic devices for various steps in cel nav, such as not using + and - for the labels to IC and instead using ON or OFF, and then once we start the sight reduction, we fall back to "If it is ON, take it OFF, and if it is OFF, put it ON."

To many of us who have in our past or present lives spend much time each day working with numbers, equations, etc this can seem an oversimplified procedure that is designed for other people.... i admit to this attitude myself when i first learned the subject.

However, after spending some time at sea in small vessels in varying conditions, one learns the value of such procedures — for all navigators, regardless of landbased experience with numerology. At sea in a small boat, we can end up very tired, and maybe on top of that not feeling very well, and we might be working in poor conditions of light and motion... in these cases (rare as they might be, since conditions at sea are more likely good than bad), then any trick at all that will help prevent mistakes is valuable. Prudent preparation covers the odd circumstances in stride, not just the routine circumstances. It is the odd circumstances that create the largest nav challenges and reward the better prepared navigators.

Such is the use of work forms in general. The idea (strange as it sounds) is to remove as much thought as possible from the process. So no matter how we might feel, we can simply plow through the process and end up with a fix. When tired it is easy to mix up a + and a -, and if we do that with the IC, we end up with twice the error. Since the IC is on the order of a minute or two in general, this leads to an overall error of 2 or 3 miles.

This level of error is sometimes difficult to spot. Most errors in cel nav have the happy property of creating a huge error in position, so they stand out nicely, and we can track then down and fix them. Hence to protect ourselves, we have to be especially careful about those potential sources of small errors that are not so prominent, and a wrong IC application is in that category.

There are many examples of these ditties in coastal navigation as well as cel nav. Rules for compass corrections, the famous (well known to those who know it well) "60 D St" method of speed-time-distance computations, and so on.

From: Starpath, Seattle, WA

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