Hi all,
I am working through the Bob Tait CNAV and Nav Workbook at the moment.
Something I am having trouble with is determining what the heading is when converting a magnetic direction to a compass direction.

Using the below compass card as an example:

I've looked through previous threads, and the general advice is use the conversion on the card which is closest to your magnetic heading; for example a heading of 219°M would most closely align with 210°M, therefore using the 210°M correction, the compass heading would correct to 221°C.

In the Nav workbook, Exercise: True, Magnetic and Compass Direction; this works for some conversions but not all (the questions where this does not work, the difference is 1°). Is the correct method to interpolate the data and come up with a more precise calculation? Am I missing something?

Thank you in advance,
Zac

I suggest that we don't make too much of a mountain out of a molehill with this.

The errors vary 0-2 degrees, ie not much at all.

Assume that intermediate headings have an error varying as a straight line (you have no basis for presuming anything else) between the two bounding stated headings and just interpolate linearly (ie as a straight line).

So, as an example

090M = 090C (nil error) 120M = 122C (compass reads 2 degrees more than magnetic)

If we wanted to know an intermediate reading, we would observe that the error varies from 0 (090M) to +2 (120M). So, if we presume a linear (straight line) variation, that would be 2 degrees error variation for the 30 degrees (090 to 120 range). If we wanted an idea of the compass heading for, say, 105M (midpoint in the range 090 to 120), we would put that at 1 degree error and conclude that 105M would be pretty close to 106C.

If you come up with a fraction of a degree, just round it off to the nearer whole degree in the usual arithmetic process that 0.5 and above rounds to the higher value while 0.4999 (etc) rounds to the whole lower value. So, for example, if we wanted the compass heading for 100M, that would be 0 + 1/3 of 2 = 0.667 so we would call it 1 degree error and round the compass heading off to 101C.

... which is what Bob says but in a slightly different way. If it still doesn't make sense, please do say so and we'll keep varying the story slightly until it does.