Hand Bearing Compass - compensation for dip angle

[Lodesman]

So if the manufacturer doesn't bias the compass to a hemisphere, they market it as a "global" compass that can be used anywhere. What is a Global Needle?

Sounds a lot like DVD codes.

[Wotname]

Just to be clear, it really isn't a hemisphere issue rather it is a "zone" issue. The link that Lodesman provided shows the 5 major zones very well.

And the Global Needle by Suunto sounds quite interesting; I wonder if there are any drawbacks.

[mulangi]

Many thanks to all those who have shared their knowledge and experience on this topic. It is reassuring to know that I'm not "barking up the wrong tree" I'll follow up on the links and ideas provided in some of the responses.
Since posting my question, I've gone ahead and bought another Vion Axium 2 Paris Mini 2000 Compass.
FYI, before placing the order, I asked the vendor if I should specify the latitude or area of intended use and the response was "You can use this compass all over this world"
I thought, use it, yes, but will it be accurate, maybe.
It's interesting to see that those who should know about dip angle and it's effect on accuracy, don't.

[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.

[GrahamHO]

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 running etc.

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......

[cabo_sailor]

Thank you Graham,
Sorry if I misspelled the name. I have a condition that makes names almost invisible. ( It took me 6 months to learn my fiancée's name)

Nevertheless your post is most informative and brings to mind what I used to know. Getting old is not what it's cracked up to be..... Sigh


Sent from my iPhone using Cruisers Sailing Forum

[GrahamHO]

Originally Posted by cabo_sailor

Thank you Graham,
Sorry if I misspelled the name. I have a condition that makes names almost invisible. ( It took me 6 months to learn my fiancée's name)

Nevertheless your post is most informative and brings to mind what I used to know. Getting old is not what it's cracked up to be..... Sigh


Sent from my iPhone using Cruisers Sailing Forum

Hi Cabo you spelt it correctly. I've got that age condition myself at 76. Writing down those compass details helps remember them and I realize most old sailors know them well. I spelt it out for any that rely on GPS

[bill good]

Iphones & pads seem to work well as a hand bearing compass. Just a thought.

Regards

[GrahamHO]

Originally Posted by bill good

Iphones & pads seem to work well as a hand bearing compass. Just a thought.

Regards

I was boarding an Air Canada flight 3 days ago and standing on the airbridge I could see into the A320 flight deck and watch the captain doing flight checks on his iPad. Or else he was playing games.

I find that the iPad or Android compass often needs re calibrating by rolling around. They wouldn't like a wave over them either. My Xperia Z3 is meant to be waterproof but I'd likely drop it. I doubt the Coast Guard would approve them but they would give an approximate bearing. A good hand bearing compass with a sight glass gives bearings within a degree.