On a fine and chilly Friday you buy yourself a compass. A nice, old fashioned manual one with a needle that swings freely.
You take it out of the box and head out to a nice flat bit of open country not far from Melbourne. The map tells you your test target destination is 5 km due North of you. It’s a small, but obvious, water tank.
You hold the compass in front of you and rotate it – and yourself – until the floating arrow directly lines up with the big N on the scale. N for North.
You know you can walk at about 1 km in 10 minutes at a good clip, so you make a note of the time and start walking. You ensure the arrow stays on the N mark. Just after 50 minutes later, you spot the tank, glistening in the sun.
About 1 km off to your left.
Magnetic Declination
1 km is a bit of a serious ‘error’. The longer you trust the compass to take you Due North, the more that error grows.
It’s because your compass, unless adjusted, probably isn’t pointing to True North (think of that as being the North as printed on your map). In fact in most places on Earth, such a compass will not point to True North.
The reason is the Earth’s magnetic field varies from place to place…and also changes (slowly) over time. The needle on your compass is pointing to your Magnetic North. It’s lining up with the direction of the Earth’s magnetic field at that point where you are.
True North is effectively a fixed, defined point. It’s where the North-South grid lines on the map are pointing. The difference between True (geographic) and Magnetic (compass) North is called your Magnetic Declination
Here in Melbourne the Magnetic Declination is currently about 11 degrees (East/positive). This excellent website not only gives the values, but these definitions:
- “If the compass at your place is pointing clockwise with respect to the True North, declination is positive or EAST”
- “If the compass at your place is pointing counter-clockwise with respect to the True North, declination is negative or WEST”
The Key to solving this
If your compass is NOT a basic (aka cheap) one, it should come with a little key or screwdriver. You simply look up the value and use the key to rotate the NESW scale (or the needle’s base plate) by the value of your location’s Magnetic Declination…and the Compass will now point to True North. If you re-do the Water Tank exercise, you’ll now be fine.
Don’t lose the key!
Here’s a nice instruction video with a similar compass to mine:
The Declination Does Vary a Lot
The approximate current values are:
- Melbourne: 11 degrees East
- Sydney: 13 East
- Adelaide: 8 East
- Perth: -2 West (!)
And check out the current world map showing the Magnetic Declination values from Wikipedia: