Originally Posted by charliehows
an interesting discussion. According to lodesmans map my old suunto handheld (I've no idea if it has a 'global needle' or not but its quite old) has worked perfectly well in zones 5,4,3, and possibly 2 but borderline. I've never heard that there are both horizontal and vertical magnetic var.s that could affect compass performance. Learn something ever' day. thanks OP.
Relating to a compass there is 1: magnetic variation 2: magnetic deviation 3: dip angle.
Magnetic variation is the difference in degrees between True North and Magnetic North. That varies around the world. In AKL, NZ that I'm familiar with, magnetic north is approx 20*east of true north and changing I think a couple of degrees a year.
It is marked on the chart compass rose along with the date of the chart and the variation on the date the chart was made.
Magnetic Deviation is the effect of the surroundings on the compass. For example with the engine
Those two are each horizontal but different from one another.
Then there is vertical dip angle which is a correction built into the compass. For the relevant hemisphere and as I understand zones. That correction prevents the compass card sticking as has been discussed.
Years ago I remember helping with the compass deviation correction on 4 engine
planes. We would go down the runway away from buildings and the plane would run all 4 engines. Then with a hand bearing compass on the ground we would line up the nose and tail from a distance in front of the aircraft and radio
our bearing reading back to the flight deck
. We would do that all around the compass turning the plane each time. I'm sure most sailors know that as swinging the compass. A correction chart was made and when the plane was flying that correction for Deviation would be combined with Variation which is a global correction depending where you are. Then both the corrections are combined to find a true course.
You can do the same in your boat
standing above the tiller with a hand bearing compass and taking a bearing on your mast
. The difference between that and your steering
compass will be the deviation of the steering
compass which is usually closer to the engine etc. Both compasses will be subject to the same Variation which generally depends on your longitude but is shown on each chart.
Early Antarctic explorers I have read about used a type of vertical compass to find the location of the South Magnetic Pole. When it pointed down they were there. In fact they correlated lots of readings in different locations. No doubt the same was done in the Arctic.
But we don't need to worry about all that stuff anymore as we have GPS
.......until it fails.
Just like we don't need morse code anymore. Except maybe dit dit dit dah dah dah dit dit dit......