<My source, with the formula's explanation and examples, is here:

http://tinyurl.com/6wff3nx It's called the One in Sixty Rule and it's evidently used by pilots.

My quick and dirty formula is: ( [distance-traveled] X [degrees-of-error] ÷ 60) = distance "off" (right or left) when you've traveled that distance.

For example: I want to travel 500 meters to my attack point, and my bearing should be 140 degrees. But I actually walk 500 meters on a bearing of 142 degrees. When I've traveled 500 meters, how far from my attack point (right or left) will I be?

500m X 2

^{o} = 1000

1000 ÷ 60 = 16.7m "off" (In this case, my error would be to the right of my attack point. Had I walked a bearing of 138 degrees, my error would be to the left of my attack point.)

Using 60 is about 90% accurate. But it's much easier to use and remember 60 than using the actual value of 57.3.

A more precise one would be ( [distance-traveled] X [degrees of error] ÷ 57.3) = distance "off". So:

500m X 2 = 1000

1000 ÷ 57.3 = 17.45m "off" (right or left).>

Thanks for this, Brian!

I now understand the maths but is it of any practical value to a walker? I'm trying to think of circumstances in which it might be used. We are all familiar with aiming off to pick up a collecting feature which then becomes a handrail which will take us to our destination or the end of a leg. It seems to me that these techniques combined with pacing, timing and a square search would cover most situations.

When I mentioned that the magnetic declination angle is small in Britain I meant in comparison with the United States. I did not mean to imply that it can be ignored when there is a need to navigate accurately.

Hugh