ABC Tables and Negative Values

[Mark_S]

Hi All,

Currently at sea and the college has broken up for summer. Having exhausted the on board teams knowledge of such things I thought I'd give it a go on here (again). Using the ABC tables for the initial course for a great circle sailing (college example). Now, I can only get the answer given in the college notes when I use the ABC tables as follows;

A= tan 15/tan75= -0.07180 (S)
B=tan 45/ sin 75= 1.03528 (S)
C= A~B= 1.03528+ (-0.07180)= 0.96348

Now, the problem here is that I'm almost certain that I remember being told to ignore any minus signs that might appear during working these out mathematically, but the only way to get the answer given in the notes is to include them. Obviously I have every faith in my lecturer not to include wrong answers in his examples (...:S) but I've a feeling something isn't quite right in this case. What made me think this more is that if you're using Nories for ABC table work no negative values are used.

Anyways if someone could help me out that would be brilliant,

Cheers!

[alistairuk]

I think you have miss understood slightly...

You can ignore the signs for this calculation...

For C..

If both A & B are the same (i.e.: both South in this case) you subtract B from A.
If A & B are different (i.e.: one is North & the other South) you add them together.

Obviously if C is then -ve, you ignore this as well, as you are writing it in the format of N/S 000 E/W normally. (N/S being named same as your final Latitude, E/W being whatever your D'Long was).

[Mark_S]

Same, Sum. Different, Difference. No?

[tlloyd1983]

C naming convention, is same name add, different name subtract. C is named greater of the two.

You could always just use the Cosine formula, and forget about all the naming conventions of ABC tables, you will also get a more accurate course this way.

Cos Course = ( Sin Latitude 2 - (Cos Distance x Sin Latitude 1) ) / (Sin Distance x Cos Latitude 1)

[tlloyd1983]

Maybe worth noting, that you always work with absolute values, so for example A = 0.2564S, B= 0.4568N, 0.2564 - 0.4568 = -0.2004. We ignore the negative, giving us C = 0.2004. We then name it greater of the two, so N, so C = 0.2004N