Introduction
This article presents facts about the type of compasses used in ordinary land navigation. Compasses used in competitive orienteering are not included.I will go through the most important parts and features of the compass. I will only explain what the features are used for. If you also want to know how to use them, you can read the land navigation tutorial.
I have also written a magnetic declination tutorial where I explain what declination is and how to compensate for it.
Types of compasses
That said, this article only deals with compasses intended for ordinary backpacking adventures and not for competition purposes.These so-called baseplate compasses (see below) are conveniently divided into the following two subcategories.
- standard baseplate compasses without sighting mirror
- baseplate compasses with sighting mirror
Parts and features of a compass
This figure shows some of the parts and features of a basic baseplate compass.
Baseplate
The baseplate is a thin rectangular piece of transparent plastic.The size of the baseplate varies between different types of compasses. The baseplate of a regular compass without a mirror could have a size of about 4 x 2.3 in. / 10 x 6 cm. The baseplate of compasses with a mirror is sometimes a little smaller.
Rulers
There are essentially two types of rulers: map scale rulers and ordinary rulers.Map scale rulers
Map scale rulers are used to very efficiently and easily measure distances on a paper map.For it to work, the map scale ruler must be compatible with the scale of the paper map. For that reason, there are usually more than one different map scale ruler on the compass.
For American USGS maps, the map scales 1:24000, 1:62500 and 1:63300 are used. The two latter maps cover larger geographical areas and are used in, for example, Alaska.
Outside America, the scales 1:25000 and 1:50000 are often used. There are also other map scales, for example I myself own a map with a scale of 1:100000 that covers a large national park in the far north of Sweden.
The scale 1:25000 means that a certain distance on the paper map is 25000 times longer in reality. This real length can be read directly on the map scale ruler.
It is quite common for a manufacturer to have two different variants of the same model: one for USGS maps and one for other maps. These compasses usually have two map scale rulers.
However, there are several examples of compasses that are compatible with both USGS and other maps. These compasses typically have three or four map scale rulers.
On some compasses, the map scale rulers are right-angled, also referred to as romer scales. Such rulers are used to easily determine a position within a certain square on the map grid.
Ordinary rulers
A compass usually also has one or two ordinary rulers to measure distances on the map. The units are either imperial or metric or both.To find out the actual distance, the length measured on the paper map needs to be multiplied by the map scale. That multiplication is not needed when using map scale rulers.
Lanyard
The remaining short edge has a hole used to attach the lanyard.The lanyard is important - you can use it to securely attach the compass to your clothing, e.g. around the belt.
Some lanyards have scales printed on them. These may be used to measure distances on a paper map.
Direction of travel arrow
There is a direction of travel arrow close to the short edge in front of the housing. It is pointing away from the compass.The arrow is used to point out the travel direction towards the desired target.
Magnifying glass
Most compasses have a magnifying glass in the middle of the base plate.The magnifying glass is used to improve viewing of map details.
Housing/Capsule
Closer to the bottom of the baseplate, there is a circular housing. The housing is sometimes referred to as the capsule.Magnetic needle
There is a magnetic needle on the inside of the housing. The middle point of the magnetic needle is anchored to a pivot in the housing.The needle is colored either red and black or red and white. The red half of the needle points towards the magnetic north pole and the other half points towards the magnetic south pole.
Compass fluid
The housing is filled with a special kind of fluid, often referred to as the compass fluid.The fluid is used to dampen the rotation of the needle and make it more stable. Without the fluid, the needle would be unstable and not settle down, making it more difficult to navigate.
Poor construction or careless handling of the compass can result in a cracked housing. The fluid will then start to leak out from the housing and annoying bubbles are created on its inside. The bubbles impair the damping ability of the fluid. The process may be slow, but sooner or later the bubbles will become so big that the compass becomes useless. I have learned this the hard way. The housing of one of my compasses cracked and it took about six months for the housing to lose 90% of its fluid, leaving a useless compass behind.
Rotating bezel/Azimuth ring
There is a rotating bezel, also referred to as the azimuth ring, on top of the housing.The bezel is scaled with degrees from 0 to 360°.
The bezel is used to set the bearing.
Bearing index
The bearing index is a marker that is either located underneath the azimuth ring (see figure 1 above) or directly on the baseplate just outside the housing. In both cases, the bearing index is fixed and does not rotate together with the bezel/azimuth ring.The bearing index is used to mark the set bearing.
Orienting arrow
There is an orienting arrow inside the housing. The arrow follows the rotation of the bezel.The orienting arrow is used to support the navigation towards the desired target.
In addition, the orienting arrow is used to decide which way to rotate the bezel - clockwise or counterclockwise - when taking a bearing on a map. The bezel shall be rotated such that the orienting arrow points towards north on the map.
Orienting lines
There are also some parallel orienting lines inside the housing. The lines follow the rotation of the bezel.The orienting lines are used together with the map to take the bearing by rotating the bezel until the orienting lines are parallel to the vertical gridlines on the map.
The orienting lines are colored red and white or red and black.
Luminous markings
Many compasses come with luminous markings.The markings are used to illuminate the vital parts of the compass in darkness.
The luminous markings are typically located on the following four places. One circular marking is located all around the bezel to make the azimuth markings visible. A second marking is located on the red half of the magnetic needle. A third marking is located around the orienting arrow inside the housing. Finally, there is one marking in front of the arrow that points in the target direction.
Rubber grip
A compass could have a rubber grip around the bezel.The rubber grip is used to get a better grip when rotating the bezel.
One of my compasses has such a rubber grip and I am very happy with it.
Sighting mirror
Some compasses are equipped with a sighting mirror. These are sometimes referred to as sighting compasses.
The mirror is used to improve accuracy, especially in wide areas.
One use case is to take the bearing from an adjacent object. Another use case is triangulation to locate yourself. A third use case is to improve accuracy in ordinary navigation.
Please see the land navigation tutorial for more details.
Clinometer
A clinometer is an arrow located inside the housing.The clinometer is used to measure slope angles.
Adjustable declination
Adjustable declination is a feature that you can use to compensate the compass for declination. A screwdriver and a small screw on the bezel will do the job.Please note that the adjustable declination feature is not mandatory. It is possible to compensate for declination also without this feature. You just need to adjust the set bearing by rotating the bezel a corresponding amount of degrees in the opposite direction. Please believe me, I have done it so many times!
Global needle
A global needle is a magnetic needle that has been balanced for usage all over the planet.From a navigational point of view, the earth is divided into geographical zones. In some cases there are three or five zones. In a more simple case, the earth is just divided into the northern and southern hemispheres.
These zones will have an effect on the compass. A compass without a global needle is only balanced for one zone, e.g. the northern hemisphere. In order to get acceptable accuracy all over the planet, you can use a compass equipped with a global needle.
Selectable features
Some of the features mentioned in this document are mandatory in all compasses. Others are optional and need to be considered before deciding which compass to choose.The following features are optional:
| Optional features |
|---|
| Lanyard |
| Magnifying glass |
| Luminous markings |
| Rubber grip |
| Sighting mirror |
| Clinometer |
| Adjustable declination |
| Global needle |
How to navigate
Please check the land navigation tutorial to learn how to navigate and to use the above mentioned features.How to compensate for magnetic declination
Please check the magnetic declination tutorial to learn what magnetic declination is and how to compensate for it.Back to Gear