December 7, 2010

Normal and natural errors

ONE OF THE THINGS that small-boat navigation teaches you is that nothing is precise. The best navigators make allowances for the unknown factors that always affect small boats, especially sailboats. They plot their positions within a circle of uncertainty and if they’re seeking landfall at a particular spot on a coastline, they aim way off to one side or the other, so they know which way to turn when they sight land.

When I was a lot younger I thought I knew how to navigate with precision. This misconception was confirmed when I sailed a 17-foot dinghy across the English Channel from Dover to Calais. I studied the tide tables and figured out the speed and direction of the tidal stream (or the set and drift as I used to call it then) for every hour. I then drew my course on the chart and adjusted the compass heading to account for the distance the tide was pushing me sideways in each hour. And thus, with a great sense of triumph, I arrived off the rather featureless French coast exactly at Calais.

It was beginner’s luck, of course. Nobody can forecast the exact speed of the current, or its exact direction. Nobody can tell you how much leeway your boat will make. Nobody can forecast your exact speed or distance covered during any one-hour period, and so the detailed markings you make so carefully on the chart turn out to be impractical nonsense. In my case, it was probably a matter of all the errors canceling each other out — a minor miracle in other words.

Years later, when further experience had taught me some humility, I read The Yacht Navigator’s Handbook, by Norman Dahl. [1] In the introduction he says: “When I was first taught navigation (in the Royal Navy) errors were thought of as being rather disgraceful, the sole result of poor technique by the navigator. Whilst I always accepted (and still accept today) that I was not the most brilliant navigator in the world, I was disappointed to find that, however hard I tried, errors never seemed to go away. Navigating a submerged submarine, and later, yachts of many kinds in many situations, eventually made me realize that errors are an integral part of navigation and need to be studied in their own right.”

Dahl said a major purpose of his book was to show that errors in navigation are normal and natural, and that a major skill in navigation lies in your ability to interpret the results in terms of the likely errors. He goes on to show boat navigators how they can actually use the errors to help make sensible decisions about their positions and a future course of action.

As one who had never experienced any difficulty in making errors I found Dahl’s advice very comforting, and I never again tried to do anything as impossibly precise as maintaining a rhumb line from Dover to Calais.

I expect Dahl’s book is out of print now because it was first published in 1983, before the great revolution in navigating that finally did bring near-precision to position-finding. We don’t think of errors now, because GPS doesn’t allow for that. It will tell us our position to within a boatlength in any kind of visibility, day or night.

And yet people have run aground using GPS, often because GPS is more accurate than the charts you plot your position on. There have been many reports of yachts wrecked on rocks, reefs, and islands that were where either GPS or the charts said they weren’t.

So we now all find ourselves in the position that I was in all those years ago, when I knew precisely and without doubt how to cross the English Channel. It’s surely time we started doubting again. It’s time we listened to Mr. Dahl, time we started taking all the possible errors into account. Time to accept that navigation is never precise, even with GPS.

[1] The Yacht Navigator's Handbook, by Norman Dahl (London, 1983, Ward Lock Ltd.)

Today’s Thought
The greatest of faults, I should say, is to be conscious of none.
— Carlyle, Heroes and Hero-Worship: The Hero as Prophet.

Boaters’ Rules of Thumb, #130
Lines of position. Two lines of position plotted on a chart should cross as near to 90 degrees as possible. For an acceptable fix, the angle between the two lines should never be less than 60 degrees or more than 120 degrees. But the old rule of thumb is that any bearing is better than none. In practice, a fix from two lines intersecting at an angle as small as 30 degrees can be used if applied with a large dose of caution and common sense about errors. Anything less than 30 degrees or more than 150 degrees is hardly worth plotting, though.

“What made you marry old Bella?”
“She was different from all the other girls I’ve met.”
“In what way?”
“She liked me.”

(Come by every Monday, Wednesday, Friday, for a new Mainly about Boats column.)


Mike said...

When I look at the advertisements for GPS sets chart plotters etc I'm thankful that I had to learn to navigate in the days before GPS was available to the average yachtie.

The adverts seem to infer that all one needs, is to buy this particular set.

I also feel uneasy about their use of the word “TO” instead of TOWARDS, those “Go To” functions seem too certain, arrogant even.

However, far from being a Luddite I wouldn’t be without my GPS now.

While I can still remember the elation of landing in France after my first solo crossing from Poole to Cherbourg in my little plywood Silhouette, I can also remember the long anxious night checking and rechecking my DR.

Ken said...

The thing about GPS that I'll surely have a mistrust of, when I do get back "out there" cruising, is that it constantly tells you where you are.

In all my past experience navigating small boats, I never cared so much as to exactly where I was, just more concerned as to where I was not, near rocks, shoals or any other hazard.

Completly trusting GPS will not come naturally to me...that'll be a good thing.

The Unlikely Boatbuilder said...

So true. I just finished sailing my Tom Gilmer "Blue Moon" up the east coast, using the GPS/chart plotter on my iPhone. I discovered two things:

1. the charts -- particularly that red line that supposedly marks the center of the ICW channel -- are mostly right, except when they aren't. When they aren't, following the chart plotter will give you an unpleasant surprise.

2. In some really twisted parts of the ICW, you need both the detailed picture a gps/chart plotter gives you, PLUS the 'big picture' view a paper chart gives you.

I got TOTALLY turned around and 'lost' in a complicated section of the NJ ICW. I was following the GPS through a series of tortuous channels in the middle of a very wide, but shallow bay. There were markers all over the place, showing different channels that went this way and that. Every time I tried to get from A to B, I'd run aground, even though that was 'impossible', according to the chart plotter.

After three tries, I anchored, made a cup of tea, put my GPS away (I'd totally lost confidence in it), and started studying the paper chart. In a minute, I saw my error. The chart gave me what I can only describe as the 'big picture' view of the bay, and then my error was obvious.

Luckily, by then I'd learned the most important lesson the ICW can teach you: run aground slowly!

I enjoy your blog!