Gridded Charts Q
 

I posted up the following Q recently and gratefully got a reply from, I think 'Send Clowns', here is that Q:

Q: A Northern L.C.C is overprinted with false grid. Constant of cone of chart is A. At B degrees West the grid track is C and the true track D. At which longitude is the false grid aligned?

The below is CS's reply which I found very helpful. However being dim I don't understand the part that says 'also whether E or W'. I would appreciate further information/explanation with respect to that part of the explanation and suggestions for a diagram to aid visualisation. Thanks in advance.

Ans: Comparing C and D will give you the chart convergence, and also whether E or W. On a Lamberts chart, convergence = change of longitude (from the datum) x sin of parallel of origin (or 'n' factor). Therefore, change of longitude = chart convergence/'n' That change of longitude is the difference between B and the datum.

You now have to work out which sense to apply it . In the N hemisphere, if convergence is W, B is E of the datum. In the S hemisphere, if convergence is W, B is W of the datum.

When you compare the Grid Track and the True Track, if the True Track is a larger number, then Chart Convergence is West. If True is smaller, Chart Convergence is East.

So Convergence West - True Best
Convergence East - True Least

With this information, you can work out whether the longitude at B degrees West (in your example) is East or West of the Datum. You need to know this, otherwise you may apply the change of longitude the wrong way.

If convergence is west, B will be west of the datum in the southern hemisphere and East of the datum in the northern hemisphere.

If convergence is east, B will be east of the datum in the southern hemisphere and West of the datum in the northern hemisphere.

Well, nobody said it was supposed to be easy!!!