I have taught map & compass for over 25 years, and never really knew there were different magnetic zones. If I were a round-the-world sailor I suppose I’d know that, but guiding in South America I only learned there were northern and southern hemisphere compasses. Not until I stumbled upon an article in New Zealand’s Wilderness Magazine (February 2009) on compasses, did I start to understand a bit more.
The article stated “Compasses are made to suit different geopgraphical zones. NZ and Australia fall into zone 5, the US in zone 1. If the wrong compass is used in any one zone, it can cause the needle to tip and drag on the base plate making for inaccurate or impossible readings. Global models that allow for these variances are available.
I read this after I had purchased what I thought was “southern hemisphere compass”. I returned the compass with article in hand and asked the store clerk if he was familiar with this. No. So we looked it up on the internet there in the store and found the following…
Before you can fully understand a “Global Needle”, you must understand a few basic principles of a compass. In order to get an accurate reading from a compass, the compass needle needs to be “balanced” in the capsule, so it does not drag on the top or bottom of the capsule. But, because the horizontal and vertical components of the earth’s magnetic field vary considerably in different locations, a compass needle that “balances” perfectly in New Zealand will drag or stick in Europe or North America for example. As a result of these magnetic variances, the compass industry has divided the earth into 5 “zones”, as identified in the map which shows the different zones starting with Zone 1 at the top and ending with Zone 5 at the bottom (Australia and New Zealand only). All of the standard compasses sold at New Zealand are balanced for Zone 5. A typical feature for regular compasses is that they can function without problems only at limited longitudes because the changes in the magnetic field affect the position of the needle. For fast needles, this phenomenon is even more prominent. In the global compass, this problem has been solved with a structural innovation. The needle and magnet are built as separate units functioning independently from each other, so that the inclination of the magnetic field cannot tilt the needle. The needle can no longer move vertically. It is the compass magnet, separated from the needle, which absorbs the vertical force of the magnetic field. The needle itself is fixed at the lid by means of a double jeweled bearing. The magnet rotates with its jewel bearing on a pin. Such a compass works reliably in all zones of the world. Due to the strong magnet, the needle settles very quickly and stops immediately at the right position, allowing for an extremely accurate reading. In addition, the global needle’s unique ability to handle tilts up to 20 degrees makes it perfect for trampers who don’t want to break their stride. Not having to level the compass exactly makes it easier to take an accurate reading while you’re still moving along the track. Source: http://www.mapworld.co.nz/global.html