Boy Scout Troop 501

All spiders are venomous… Here’s a handy little chart to help distinguish the nasties from those harmless to humans.

This is a very easy lesson, and not sufficient for those who would like to travel safely in unfamiliar terrain.

The first thing you need to learn, are the directions. North, South, East and West. Look at the figure and learn how they are. North is the most important.

There are several kinds of compasses, one kind to attach to the map, one kind to attach to your thumb. The thumb-compass is used mostly by orienteers who just want to run fast, and others. Let’s take a look at a standard map compass:

But if you don’t want to go north, but a different direction? You’ve got this turnable thing on your compass. We call it the Compass housing. On the edge of the compass housing, you will probably have a scale. From 0 to 360 or from 0 to 400. Those are the degrees or the azimuth (or you may also call it the bearing in some contexts). And you should have the letters N, S, W and E for North, South, West and East. If you want to go in a direction between two of these, you would combine them. If you would like to go in a direction just between North and West, you simply say: “I would like to go Northwest “.

Let’s use that as an example: You want to go northwest. What you do, is that you find out where on the compass housing northwest is. Then you turn the compass housing so that northwest on the housing comes exactly there where the largedirection of travel-arrow meets the housing. 
Hold the compass in your hand. And you’ll have to hold it quite flat, so that the compass needle can turn. Then turn yourself, your hand, the entire compass, just make sure the compass housing doesn’t turn, and turn it until the compass needle is aligned with the lines inside the compass housing.

Now, time to be careful! It is extremely important that the red, north part of the compass needle points at north in the compass housing. If south points at north, you would walk off in the exact opposite direction of what you want! And it’s a very common mistake among beginners. So always take a second look to make sure you did it right!
A second problem might be local magnetic attractions. If you are carrying something of iron or something like that, it might disturb the arrow. Even a staple in your map might be a problem. Make sure there is nothing of the sort around. There is a possibility for magnetic attractions in the soil as well, “magnetic deviation“, but they are rarely seen. Might occur if you’re in a mining district.

When you are sure you’ve got it right, walk off in the direction the direction of travel-arrow is pointing. To avoid getting off the course, make sure to look at the compass quite frequently, say every hundred meters at least. Don’t stare down on the compass. Once you have the direction, aim on some point in the distance, and go there. But this gets more important when you use a map.

There is something you should look for to avoid going in the opposite direction: The Sun. At noon, the sun is roughly in South (or in the north on the southern hemisphere), so if you are heading north and have the sun in your face, it should ring a bell.

When do you need this technique? If you are out there without a map, and you don’t know where you are, but you know that there is a road, trail, stream, river or something long and big you can’t miss if you go in the right direction. And you know in what direction you must go to get there, at least approximately what direction. Then all you need to do, is to turn the compass housing, so that the direction you want to go in, is where the direction of travel-arrow meets the housing. And follow the above steps.

Why isn’t this sufficient? It is not very accurate. You are going in the right direction, and you won’t go around in circles, but you’re very lucky if you hit a small spot this way. And that’s why we’re not talking about declination here. And because that is something connected with the use of maps. But if you have a mental image of the map and know what it is, do think about it. But I think you won’t be able to be so accurate so the declination won’t make a difference.

If you are taking a long hike in unfamiliar terrain, you should always carry a good map that covers the terrain. Especially if you are leaving the trail. It is in this interaction between the map and a compass, that the compass becomes really valuable. And that is dealt with in lesson 2.

Using a Lensatic Compass

An Engineer’s or Lensatic compass maintains a primary function of taking accurate bearings for land navigation. An additional function of this compass is to direct artillery fire. For these reasons, it is a highly coveted piece of equipment used by the military. Standard Lensatic covering is square with one side ruled. This is to aide military personnel in triangulating known landmarks to gauge positioning.The compass is comprised of three major components: the cover, the base and the lens. The cover serves the purpose of protecting the compass rose dial. It contains sighting wires and dots utilized in nighttime navigation. The base is where all movable parts of the compass reside. A floating dial rotates indicating direction each time the compass maintains a level position. Incandescent figures depict the directions east (E) and west (W). In the center lies the directional arrow. This always points in the direction of north (N). East falls at 90° and west at 270°. There are also two scales. The outer scale denotes miles and the inner scale denotes degrees. The inner scale indicator is in red color.

A bezel ring is also inside the compass base. This ratchet device turns 120 clicks on a full rotation. Each individual click represents 3°. A short incandescent line works with the north-directional arrow in navigation. This line lies in the glass face of the bezel ring. The floating dial contains a fixed black index line as well. The final component of the base is a thumb loop. This simply attaches to the base as a handling mechanism.

The last component of the Lensatic compass is the lens. This is what reads the floating dial. A rear-sight navigational slot works with the front sight wires in the cover to locate objects. In addition, this slot protects the compass when in the closed position. This works by a lock and clamp system. The rear-sight mechanism must remain open at a minimum of 45° in order for the compass floating dial to work.

To Take a Bearing

1. Unlock the cover half way so that the compass card forms a 90° angle.
2. Raise the lens arm to a 45° angle.
3. Stabilize the compass by placing thumb inside the thumb hook. Ensure the hook is all the way towards the bottom before doing so.
4. Locate the target object.
5. Adjust the sighting wire so that it lies in the center of the target object.
6. Read fine degree marking on the compass card. Do this by moving the lens up and down until the degree mark reads without taking your eye off the target.
7. Read the bearing in degrees or MILS, whichever is preferred.

Set a Bearing

1. Follow the directions above.
2. Bring the marking on the bezel in line with the north direction arrow. Indication of this comes off the compass card.
3. Once the bezel marking and directional arrow align, orientation is set.
4. Proceed on course in direction indicated by sighting wire.

Follow a Bearing

1. Follow directions above.
2. Pinpoint landmark in the distance to serve as a reference point.
3. When reference point disappears, due to weather or trees, use compass to stay the course.
4. Occasionally set a new bearing for your selected reference point.

More documents about the use of Lensatic Compasses can be found here:

How to Use a Lensatic Compass Guide

Lensatic Compass Information

Unfortunately, there is something called magnetic declination. You see, the compass is pointing towards the magnetic northpole, and the map is pointing toward s the geographic northpole, and that is not the same place. To make things even more complicated, there is on most hiking-maps something (that is very useful) called the UTM-grid. This grid doesn’t have a real north pole, but in most cases, the lines are not too far away from the other norths. Since this grid covers the map, it is convenient to use as meridians.

On most orienteering maps (newer than the early 70’s), this is corrected, so you won’t have to worry about it. But on topographic maps, this is a problem.

First, you’ll have to know how large the declination is, in degrees. This depends on where on the earth you are. So you will have to find out before you leave home. Or somewhere on the map, it says something about it. One thing you have to remember in some areas, the declination changes significantly, so you’ll need to know what it is this year.

If you are using a map with a “UTM-grid,” you want to know how this grid differs from the magnetic pole.

When you are taking out a course, you will do that more or less as described in lesson 2, 
but this time, you must also look out so that you don’t align the orienting lines with the grid lines pointing west or east, or south for that matter. When you have taken out a course like you’ve learned, you must add or subract an angle, and that angle is the angle you found before you left home, the angle between the grid lines or meridians and the magnetic north.

The declination is given as e.g. “15 degrees east”. When you look at the figure, you can pretend that plus is to the right, or east, and minus is to the left and west. Like a curved row of numbers. So when something is more than zero you’ll subtract to get it back to zero. And if it is less, you’ll add. So in this case you’ll subtract 15 degrees to the azimuth, by turning the compass housing, according to the numbers on the housing. Now, finally, the direction of travel-arrow points in the direction you want to go. Again, be careful to aim at some distant object and off you go.

You may not need to find the declination before you leave home, actually. There is a fast and pretty good method to find the declination, wherever you are. This method has also the advantage that corrects for local conditions that may be present. This is what you do:

1. Determine by map inspection the grid azimuth from your location to a know, visible, distant point. The further away, the more accurate it gets. This means you have to know where you are, and be pretty sure about one other feature in the terrain.
2. Sight on that distant point with the compass and note the magnetic azimuth. You do that by turning the compass housing so that it is aligned with the needle. You may now read the number from the housing where it meets the base of the direction of travel-arrow.
3. Compare the two azimuths. The difference is the declination.
4. Update as necessary. You shouldn’t need to do this very often, unless you travel in a terrain with lots of mineral deposits.

There are a few riddles and rhymes to help you remember whether you should add or subtract:

empty sea, add water.

eMpTy Cea, add Water

MTC, add W

Map To Compass, add West.

Which means, when translating from map to compass, add the number of degrees of west declination. Subtract east declination. When going from compass to map, just reverse, adding east declination, and subtracting west.

Uncertainty

You can’t always expect to hit exactly what you are looking for. In fact, you must expect to get a little off course.

How much you get off course depends very often on the things around you. How dense the forest is, fog, visibility is a keyword. And of course, it depends on how accurate you are. You do make things better by being careful when you take out a course, and it is important to aim as far ahead as you can see.

In normal forest conditions we say that as a rule of thumb, the uncertainty is one tenth of the distance traveled. So if it is like in the figure, you go 200 meters on course, it is possible that you end up a little off course, 20 meters or so. If you’re looking for something smaller than 20 meters across, there is a chance you’ll miss. If you want to hit that rock in our example you’ll need to keep the eyes open!

In the open mountain areas, things are of course a lot easier when you can see far ahead of you.