NavList:

If I may add.... Fixes are not just a new starting point, they are also a finishing point. There is where you analyze why there is a difference between your DR position and your fix.  Many things could cause this, compass error, etc.  Even if you can't pinpoint a cause you might notice a consistent bias in one direction or another,which you could allow for in subsequent DR.

On Jul 17, 2015, at 2:06 PM, Frank Reed wrote:

John Howard, you asked:
"What precision is required when doing cel nav ?"

The best that you can achieve... if you're actually "navigating" for a practical purpose. The limit should only be the "noise limit" of the entire process. If you can get a fix accurate to within a mile, then why wouldn't you? You might need it...

Stephen Davies already mentioned the possibility of winning an ocean race by virtue of accurate navigation. So even in a navigation "game", accuracy counts. If you're not in a race against other vessels, then, unless you're a yachtsman at leisure, you are presumably racing against time one way or another. Accurate scientific navigation, after all, was invented to make ocean navigation safer and more efficient. It was already merely "possible" centuries before the era of sextants, chronometers, almanacs, and logarithms.

It's important to remember, also, the limited availability of celestial navigation. If you are two days from a landfall, and you have a celestial fix that you have worked up indifferent to accuracy --by the very reasonable logic that you're still a long way from land-- that may be fine as long as the weather holds. You can do better tomorrow. But what if you're clouded out a few hours after that fix, and the skies remain cloudy as you approach land. If you accepted +/- 10 mile inaccuracy in your fix, then that is the minimum uncertainty as you're approaching land with the additional uncertainty in your subsequent dead reckoning compounding it. Shouldn't you take every fix with the working assumption that it might be your last for several days, at least in certain regions like the North Atlantic in stormy season? Of course, if you're not "really navigating" and celestial is just an amusement, then you can set your own rules.

If you're using celestial navigation for real practical navigation, having thrown your GPS overboard, you will still want an accurate fix, too, if you get into any kind of trouble. Deep in the South Pacific far from any landfall, if you start taking on water and put out a call for help to the team of "International Rescue" stationed on Tracy Island, then an accurate position in mid-ocean may mean the difference between survival and a slow descent to a dark and watery grave. Since the early twentieth century, fixes aren't just for us on-board; they're for communicating with the rest of the world when the need arises.

Frank Reed
ReedNavigation.com
Conanicut Island USA