Traditional Navigation: the EP and Fix - Sail Magazine

Traditional Navigation: the EP and Fix

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A triangle marks the estimated position (EP), with time, course steered, distance sailed since previous EP and log reading noted; the GPS fix indicates the EP is off by a mile; insufficient allowance for current, perhaps

A triangle marks the estimated position (EP), with time, course steered, distance sailed since previous EP and log reading noted; the GPS fix indicates the EP is off by a mile; insufficient allowance for current, perhaps

Working with a plotter or even using basic GPS with a paper chart, the fix is inevitably “king.” A GPS fix is constantly available and should be so accurate that the sharpest pencil can’t do it justice. However, before GPS, things were different. The crux then was a dead reckoning position estimated from course steered and distance run, modified into an estimated position (EP) using tide, current or leeway. Fixes were used to check or refine this.

The beauty of this approach is that you start out by knowing approximately where you should be. Should the fix turn out to be wildly at variance, you examine both positions more closely. If you don’t begin with an EP, there’s no way of knowing if a fix is OK or not. Using plotted compass bearings or ranges, adjudicating a plotted fix wasn’t hard. A tight cocked hat was probably a winner. If, however, it looked like a drunken spider’s web, you had to ask why. The trouble with a GPS fix on a paper chart is that, while the instrument is spot-on, it may be anything but by the time we’ve transferred it with dividers and ruler. If there’s no EP, we take it as read, and away we go onto the rocks. Mistakes here are often radical rather than the width of a pencil point, so an EP, even a rough one, will smoke them out if we’ve blundered.

EP and fix—a middle course for today

It isn’t realistic to expect a paper-chart navigator to plot a full-on EP every hour. It’s no hardship, however, just to log course and distance, glance at any relevant current data, then place a well-tuned thumb over an approximate position. Now plot the GPS lat/long. If the two coincide, all’s well. If not, better take another look at the fix, because you’re more likely to have mis-plotted that than you are to have muffed the EP. You ought, by rights, to be cross-checking the plotted fix against course and bearing to a waypoint as well, but the essence of the EP is that it is drawn from totally different source data, which is the crux of classical navigation.

October 2015

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