Using your Compass with a Map - Measuring Angles with your Compass
Hi, John with maptools.com here to show you how to work with bearings on your map.
The function of a magnetic compass is to measure the angle between the Earth's magnetic field and a target you have pointed the compass towards.
All magnetic compasses have a magnetized element that is free to rotate and align itself with the Earth's magnetic field. It may be a simple needle or it may be a disk printed with cardinal directions and angular measurement markings.
The most basic compasses are just marked with the cardinal directions: north, south, east, and west.
In land navigation, when we use a compass to measure the angle between magnetic north, our location, and some other location, the measurement is made in units of degrees. The full circle is divided into 360 equal parts. North is defined as the starting point for the measurement and assigned the value of zero degrees. The measured angle increases in a clockwise direction. The cardinal direction of east has a value of 90 degrees, south is at 180 degrees, west is at 270 degrees, and we return to north at 360 degrees.
Let's look at several typical compass dials.
Here's a very typical compass dial. The dial is labeled every 20 degrees. It has longer tic marks every 10 degrees and a tic every two degrees.
Here's a compass with a molded magnifier over the index line. The dial is labeled every 10 degrees has longer tic marks every five degrees, and a tic for every degree.
On the other end, here's a low-cost compass. The dial is labeled every 30 degrees, it has longer tic marks every 10 degrees, and a tic only every five degrees.
Let's try reading some bearings. On this compass, the dial is labeled every 20 degrees, longer tics every 10 degrees, and each short tic represents two degrees. The index mark on this compass is sharp, but it's hard to see because it's clear plastic. It does have a bit of phosphorescent paint which helps.
The bearing here is 296 degrees.
Your turn. What's the bearing in this image?
How about the bearing in this image?
Here's a compass with one of my favorite index lines. It's very thin and sharp, has a contrasting color, and touches the marks on the dial. The dial markings are inside the capsule so they are unlikely to wear off over time. The dial is labeled every 20 degrees, longer tic marks every 10 degrees, and each short tic is two degrees.
What's the bearing?
How about in this image?
What's the bearing here?
It's possible to read this dial with one degree accuracy, although it requires some judgment on your part.
Here's the compass with the built-in magnifier over the index line. You'll notice this compass also has a different kind of needle. Remember, on this compass, the dial is labeled every 10 degrees, longer tics are every five degrees, and each short tic is one degree.
What's the bearing in this image?
With a bit of judgment on your part, it should be possible to read this dial with a half degree accuracy. I'd call the bearing in the second image here 357.5 degrees.
Here's an example of a bad index line. Really there are two index marks. One of them is the arrow molded into the clear plastic above the dial, the other is a now very worn bit of phosphorescent paint, visible here through the center of the last zero in 300. The two index marks are not even lined up with each other. So the bearing is either 305 degrees or 306 degrees? Hard to tell.
The markings on the dial and the index line don't get much ink on the reviews or on the packaging when you go to buy a new compass, but as you have seen, they make a big difference in accuracy and ease of use.