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Friday, 20 July 2018

Compass Variation and Deviation

Compass Variation and Deviation

To convert a course to steer True (T) to a course to steer Compass (C) we must allow for both Variation (V) and Deviation (D).  A simple way to remember the order of work is by using the mnemonic 'True Virgins Make Dull Company' or 'Tele Vision Makes Dumb Children' and the word CADET (Compass to True Add East) to remind us if we need to add or subtract. Of course there are several saying about the compass, least and best and it can take a while to learn them all, maybe I am over simplifying things but all you need to know is one then the rest can be worked out easily. If you get a saying wrong you can make mistakes.

T          V             M               D               C
True     Variation Magnetic    Deviation  Compass
Tele      Vision      Makes       Dumb        Children
 
Compass to True Add East
C               AD   E    T
Compass  Add  East True

If we convert from Compass to True the word CADET is used to remind us that we add any Easterly variation and deviation (any Westerly variation or deviation would then be subtracted).

If we are converting True to Compass the reverse is correct we subtract any Easterly variation or deviation (add any Westerly variation or deviation).

While on the subject of electronics one thing I do know is that my fluxgate compass has a lot more deviation than the old fashioned steering compass mounted on the steering pedestal. However I am not sure if this is true of those pedestal mounted steering compasses with a chart plotter mounted just centimetres above them. Check the compass safe distance your compass can be from your chart plotter, you may find it’s a metre or more.

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The One In Sixty Rule

This rule of thumb can be used to work out your lateral movement from your deviation.
For instance if you are one degree off course after 60 miles you will be 1 mile from you intended track.  For every one degree off course the distance-off will be 1/60 of the distance covered. For two degrees off, then 2/60 of the distance travelled.

This only works with small distances and amounts of deviation.

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Recently we were using an electronic chart that was having trouble supplying the correct magnetic variation for display on the chart plotter. This meant I was continually working the true heading to compass and back again. Later that day while having sundowners I was then telling a couple of people about this and they started to look at me like I had two heads. I was not sure why at first but as it turned out they didn't know how to do the conversion easily so here is what I think is an easy way of remembering how to do the work through. This is not new, really I was taught this years ago. I think the reason people forget these calculations stems from the use of chart plotters and not knowing or needing to know the interworking of the machine to navigate whether that’s good or bad is purely a perception thing. However if the chart plotter fails and your left with a handheld GPS and a chart its worth knowing how to apply variation and deviation so you can work out a course to steer to get you to your destination.

How did I know there was a problem? To start with the heading and COG pointers on the chart plotter were a long way apart and it didn’t make any sense. Working through on a paper chart and using standard methods for course to steer and checking the drift rate, like looking out the back, and using tide tables we were able to work it back to an error with the chart plotter. Once a manual variation was put in to the chartplotter setup, the COG and heading pointers moved a lot closer together and when we were motoring into an anchorage with little current they were as they should be together. 

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