So, by this time, you've learned to identify surroundings by taking bearings, and to find your position using back bearings. These practices are designed to orient you with respect to a map or chart - that is to say: show you where you are located relative to your surroundings.
This series of posts deal with assessing motion and have the following topics:
Moving in a straight line (this post)
Navigation, by it's definition, implies that you're moving through an environment. If you're not following a trail, you might want to move in a straight line. There are three terms that are used in navigation, and you'll find variations on these terms: bearing, heading, and course. Here are some rough definitions:
Bearing this is the azimuth from you to an object you're sighting or have sighted
Heading this one can be trickier, depending on who you're talking to. If you're walking in a direction, it's the direction you're facing, so normally, you'd think your heading would be the direction you're walking, right? Well, a lot of times people end up walking with a slight sideways skew, and don't realize it. If you're in a kayak or a sailing vessel, the heading is the direction of the long axis of the vessel as it's in motion. A current can push the vessel sideways, in which case the direction of motion does not line up exactly with the heading. On the other hand, with a lack of wind and current, the direction of motion would be the same as your heading. For the most part, heading is taken to be the same as the direction of motion, but the above discussion can show you the subtleties that can arise. So a heading is the azimuth you face or your vessel points toward.
Course this generally means direction of motion. Implicit in the name is sometimes the idea that a course is not a straight line, but a path that's being followed. The word heading has more of an implication that a person is momentarily moving in a straight line, modulo factors like current.
Finally, there's a more specific:
Course bearing or sometimes "course heading" - this is meant to convey the azimuth that a person or vessel is moving with respect to an 'absolute' reference system. The 'absolute' system usually means land.
This is illustrated in the figure below. Say you are paddling in a kayak. The forward motion with respect to the water is defined by the line of symmetry along the length of the kayak. When you paddle, the azimuth of this direction is the heading. But suppose you are paddling under the influence of a current, as shown. Then the direction of motion relative to fixed points of land would be altered by the current and your true direction is indicated by the course bearing as in the figure.
Difference between heading and course bearing when paddling a kayak under the influence of a current.
For walkers trying to walk in a straight line, the difference between their heading and their course bearing might be related to walking on a slope that causes a sideways motion.
For kayakers, the difference might be due to both current and wind pushing the kayak sideways.
For sailors, not only does current come into play, but also leeway. Leeway is the inevitable sideways slippage that occurs then the sailboat is moving at some angle relative to the wind, typically on a reach.
Correcting for the difference between heading and determining course bearing can be important when engaged in dead reckoning. On the other hand, there are some tricks that can be employed to ensure that the heading and course bearing are the same.
Let's first take a simple case - you want to move in a straight line while walking and you have a normal backpacker's compass.
You would think that simply moving in a straight line would be easy, right? Not so. People who wander in a featureless environment, such as thick woods on a plateau with no visible reference or even wind as a natural compass often have difficulties with this. Their paths look loopy, often doubling back on them selves, convincing the walker that he or she may be walking in circles. I've personally gotten off track walking while staring at a compass. Why? When you're staring at your compass, you are not necessarily walking in the direction you want.
The are ways of combatting this. First, find the desired direction of travel as an azimuth. You may want to do this by taking a bearing on a map to your destination. In any case, once you have the azimuth, line up the compass with the direction of travel line on the compass pointing in the azimuth, and then look for objects in the distance in that azimuth. The farther away, the better, providing it lines up well. Better still, try to find two objects in a line with your desired heading.
Once you've done that, you can put away the compass and walk to the first landmark. At that point, you can pull out the compass and line up the next two objects.
Using objects that line up with your desired direction of travel to maintain a straight line of travel.
There are all sorts of variations of this theme of lining up objects in the distance to maintain a straight line of travel. It has been said that camel caravans in the sahara travel in single file, and that the job of the camel driver at the rear is to shout orders to the front to keep the camels walking in a line. It's difficult to verify this tidbit, but it would be interesting if true.
The next post deals with dead reckoning.
thanks for the explanation, nice (y)
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