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Welcome to the CPL Navigation question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.

gregbrowning created the topic: Problems working out compass deviation

Hi all,
I have a question regarding compass deviation. I've just started working through Bob's Nav Workbook and have realised that I don't know how to calculate the precise compass heading.

Obviously, when the compass deviation card says something like this:

For M: 030 060
Steer C: 028 058

I understand that the course will always be 2 degrees less. E.g. for 025 M, you need to steer 023.

However, what if the numbers are not separated by 30.
For example, in the Nav workbook:

For M: N 030
Steer C: 000 028

For 022M it says steer 021 (I wrote steer 20)
But for 027M steer 027. (I wrote steer 25)

I made many mistakes due to not knowing how to round up or down here.

bobtait replied the topic: Problems working out compass deviation

Here's another example

For M 030 060
Steer C 028 060

The compass heading is 2 under for 030 and is correct for 060. So halfway between it would be 1 under. Nobody is interested in fractions of degrees on a compass, so if the magnetic heading is closer to 030 than it is to halfway, use 2 under, if it is closer to half way than to either 030 or 060, use 1 under and if it is closer to 060 than halfway, make no allowance.