Wednesday, October 4, 2017

Magnetic Dip and Zone Balancing

Traveling from Seattle to the South Pacific?

To preserve compass accuracy when traveling large distances, the dial card in your compass should be balanced in order to compensate for the dip caused by the Earth's magnetic field. The picture below is the  back of a compass card, showing the two magnets that make it work, along with the daubs of solder on the corners that are applied to balance the card.

Here are a few related notes from the Ritchie Compass company
"Ritchie compasses come standard balanced for Zone 1, which essentially includes all of the Northern hemisphere. If you're requesting balancing for Zones 2-7, simply indicate the zone that is most central to your boating area."

"Once your compass is balanced for a specific zone, it will maintain accuracy for one Zone north or south. Ritchie recommends using a compass that is balanced for the zone where the boat will be operated most frequently."

To help you judge this more specifically, see this global plot of the dip angle (inclination).

You can also look at just the vertical component of the magnetic field as another way to judge this. A high res pdf of that, along with one of the inclination shown above, plus world variation and other useful tools is at NOAA's World Geomagnetic Data Center, which is now part of the NCEI, see especially the link to these types of images (they are high res pdfs). It could be valuable to save the pdf of world variation—they call it "declination."

Here is another company's plot of the zone balancing overlaid on one of these dip maps... at some (unknown) date in the past. The lines have slipped north somewhat over whatever time period is reflected here.


Larmstrong said...

To ask the obvious, I have a Danforth Constellation on board a boat that was originally from Florida, now in Australia and the card is tilted. This would most likely be caused by dip yes?
To fix it it has to be rebalanced for the zone I'm sailing?
So if you're sailing around the would how inaccurate and can't adjust for dip how innaccurate does the compass become? Are compass cards readily adjustable?

David Burch said...

Yes, that is likely the dip you are seeing. These can be leveled at a good compass repair store. The error is likely to depend on the direction looking. You can check it by taking compass bearings to known stars, then compute the true bearings from cel nav ie see or /calc and convert true to magnetic from known variation at that location, then compare magnetic to what the compass shows. the difference is the error. You can also use an electronic compass after removing deviation. they are not affected by such things. Cell phones have such compass apps, but you must be careful that they are very level. Small tilt throws them off.

Anonymous said...

When dial cards are gold in color (as opposed to white or maybe glow-in-the-dark), what material are they made of and is there a benefit? I am in search of a (circa 1980) gold-colored HENRY BROWNE & SON LTD Sestrel model (likely from a gyro compass) but, all of the examples I can find online appear to be white.
Who is a good resource to inquire about identifying antique/vintage makes and models?
Thank you in advance.
Continued success!

David Burch said...

You might check with Ridge White at