Compass FAQs
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want to check out our learning section back on the home page. |
What should I look for in a good compass?
How do I use a compass?
What is a reciprocal scale?
What is the compass mirror used for?
Do I need a Global Needle if I will only be in the
US?
What are the "cotangent" tables on the KB-14 sighting
compass used for?
What should I look for in a compass?
Accuracy and durability should be paramount in your quest for a good
compass. The easiest way to determine reliability may be the brand
(The most dependable brands are Suunto, Silva, and Brunton). While
the brand is definitely a good indication of quality, each manufacturer
supplies compasses of varying grades including various features.
When it comes to choosing a specific compass you will want to consider
the following:
- Bezel resolution (5 degree, 2 degree, 1 degree, 1/2 degree -- lower is better)
- Compass accuracy (again lower is better)
- Dampening time (how long it takes the needle to settle)
- Capsule temperature range (don't let the cold weather get you lost)
- Translucent baseplate? (this is helpful when working with maps)
- Sighting mirror? (this feature can allow more accurate readings to be taken)
- Declination scale? (helpful for quick declination calculations)
- Adjustable declination? (eliminate the need for calculation altogether)
- Clinometer? (for determining slopes or heights)
- Reciprocal Scale? (very helpful for sighting compasses. not necessary for others)
What is a reciprocal scale?
What is the compass mirror used for?
Do I need a Global Needle if I will only be in the US?
What are the "cotangent" tables on the KB-14 sighting compass used
for?
(cotangent of 4 degrees) X (1/2 mile) = your
aprox distance to the house or mountain.
Figure A is a more accurate depiction of the proper geometric properties,
but it is a little unrealistic to assume you can determine the distance
of that cotangent line with great accuracy, since you don't know your own
exact location. Figure B is a more realistic example of how to use
a cotangent to approximate your distance to a destination.
How do I use a compass?
We have developed an entire page on how to use a map and compass.
If the extent of your knowledge stops at "the needle points North", you
should be sure to check our our "How
To Use A Map & Compass" page. And once you begin to work
with a compass, you will quickly want to learn
The reciprocal scale is often called the reverse scale, because the reciprocal direction is the exact opposite of the direction you are heading. The reverse/reciprocal scale is very useful in determining your location
on a map. For example: if you see that mountain top "A" is at a bearing
of 240 degrees from you (the reciprocal is 60 degrees), and the bluff is
at a bearing of 300 degrees from you (the reciprocal is 120 degrees), you
can draw a line on your map from the mountain top "A" in the direction
of 60 degrees, and a line from the bluff at 120 degrees. Where these
lines intersect is the location you are standing. Reciprocals are
easy to get from a baseplate or mirror compass, because you just look at
the white tail of the needle and read the reverse direction. Since
you can't do that when looking through a sighting compass the smaller reciprocal
scale is very helpful.
The compass mirror allows the user to view the target and the compass
capsule at the same time. This is why we group mirror compasses in
the sighting compass family -- as you can "sight" your target and your
bearing at the same time. See our compass
mirror page for more details.
Without a doubt, the greatest benefit of the Global Needle is the capability
of it's worldwide use. As a result of the design however, there is
a secondary benefit. The way the Global Needle is capable of handling
worldwide magnetic zones is with it's ability to handle needle tilts of
up to 20 degrees. This characteristic is great for hiking because
it means as you bounce along the trail, you don't need to have the compass
perfectly level to get an accurate reading... making it easier to
get readings while you are walking. To learn more about the Suunto's
patented Global Needle, see our "What
is a Global Needle?" page.
The cotangent table is helpful for determining your distance from specific objects. First, lets talk about Figure A. Assume that with the map, you
can determine the relationships between the location of the house and the
tree, but you aren't sure your exact location. Use the map to determine
the distance of that short perpendicular line from the house to the line
going towards the tree. If you take that distance and multiply it
by the cotangent of 20 degrees (the angle you determined with your compass),
that will tell you distance between you and the house. And since
you probably don't know the cotangent of 20 degrees by memory, you just
have to refer to the handy cotangent table on the back of your KB-14.
Figure B show a more realistic and typical situation. First,
you would pick two objects in the horizon that seem as best you can tell
to be roughly side by side. If you can determine the angle between
the two objects with your compass (let's say it was 4 degrees) and you
know from your map that those two objects are 1/2 mile apart, then you
just perform this equation:
...and since the scale on the KB-14 says the
cotangent of 4 degrees is 14.30, you can calculate...
(14.30) x (1/2) = 7.15 miles to the mountain.
