Traditional Navigation: the EP and Fix

Author:
Publish date:
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

Related

7261ab1f-6891-424f-a22f-14c946c08ba8

Gear: Fusion Panel-Stereo

Plug & Play StereoIt can be a real pain to install a marine stereo inside a boat, what with the tiresome business of running cables through cramped spaces and finding somewhere sensible to locate the speakers. The audiophiles at Fusion thought about this and came up with the ...read more

2019BestBoatsPromo-04

Best Boats 2019

Some years ago, the book Aak to Zumbra catalogued—and celebrated—the incredible diversity of watercraft that has evolved over the centuries, a diversity that remains evident to this day in the 11 winners comprising the “Class of 2019” in SAIL’s Best Boats contest. Indeed, it ...read more

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell.Got a tip? Send it to sailmail@sailmagazine.comGuaranteed result What you see on the end of this halyard isn’t a beautiful Flemish Eye worked by a rigger, but it will make a big difference when you have to “mouse” a line through the mast. If the ...read more

dometicadler-700x

How to: Upgrading Your Icebox

The time has come when the prospect of cold drinks and long-term food storage has you thinking about upgrading your icebox to DC-powered refrigeration. Duncan Kent has been there and done that, and has some adviceFresh food must be kept at a refrigerated temperature of 40 degrees ...read more

Jet-in-Belize

Cruising: Evolution of a Dream

There’s a time to go cruising and a time to stop. As Chris DiCroce found, you don’t always get to choose those timesAlbert Einstein said, “Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, ...read more

01a-rosemary-anchored-at-Qooqqut,-inland-from-Nuuk

Cruising: A Passage to Greenland

When a former winner of the Whitbread Round the World Race invites you to sail the Northwest Passage, there is only one sensible answer. No.More adventurous types might disagree, but they weren’t the ones facing frostbite of the lungs or the possibility of having the yacht’s hull ...read more