Plotting

Does plotting have to be done on paper, or can we do it in our heads? The fact is that most recreational and light commercial mariners do most of their plotting in their heads. During training of candidates for a captain's license, we instruct mariners on how to put their "plotting" thoughts on paper. Let's talk about that.

If I asked small craft operators how often they used a "line of position" in their every day navigation, most of them would say that they have never done that. However, it is a near certainty that they all have used that technique.

A "line of position" represents an infinite series of possible positions at which your vessel may be located. It can be drawn on your chart based on a compass bearing to a landmark. In the past, we have discussed drawing such a line at the angle indicated by your bearing. The resulting line, on which your vessel is somewhere situated, would run through both the landmark and your current position.

Things need not even be that complicated. For example, if you are transiting a river or canal, you can observe a landmark without the benefit of a bearing. If you are 500 yards off the bank in the vicinity of a landmark that is shown on the chart, do you really need a compass bearing to determine your position? Clearly, you have conceptualized a line of position, albeit approximate.

So, every time we use a landmark to determine how far down the coast we have run, we have conceptualized a line of position. But, how about "dead reckoning?" Can we do that in our heads also? You bet we can, but first let's define it. Dead reckoning uses a known position (a fix) as a starting point. Based on a course and speed from that fix, we plot a line that indicates the direction and distance that we will travel in a specific period of time.

How about a dead reckoning plot to cross a four-mile wide river in restricted visibility? Assume we are leaving a harbor channel and intend to enter the mouth of a small creek on the other side of the river. Also assume that due to the poor visibility, you will not be able to see the first "aid to navigation" at the mouth of the creek until you are half way across the river.

First, from your chart, you determine the course from the harbor channel (your known position) to the creek. You've done that sort of thing many times. Next, head toward the "obscured" channel marker at the entrance to the creek. Eventually, you'll spot the first channel marker in the creek. You just did a dead reckoning plot in your head.

At the bottom of all of this is that we often end up "plotting on the fly" or "shooting from the hip" in order to move our vessel from point to point. The key to achieving the appropriate level of diligence in plotting is to know the consequences of your plot being wrong. The higher the risk, the more serious you have to be. And that brings us back to that old commodity known as "common sense."

Until next time, I wish you clear skies, fair winds, and calm seas!