NavList:

Hello, Martin.

Welcome back! It's been a while, and I was starting to worry you might have decided to stay in the UK following your Trans-Atlantic celestial adventure.

You wrote:
"Sincerely, I am not sure what the advantage would be in using the dead reckoning latitude and longitude (rather than the assumed ones) when doing the sight reduction. The intercept and azimuth will be different, but the resultant lines of position would intercept on the same point. I leave this question open for discussion."

It's an interesting question. I have never used 214, but clearly it's only slightly removed from 229, which, if I'm not mistaken, was produced with the intent of replacing 214. So what's the advantage of working from your DR position instead of some other Assumed Position? First note that this still qualifies as an AP. It's not an AP with special rules about integer latitude and integer longitude difference (usually called integer LHA). Many modern celestial navigators believe that the "special rules" are part of the definition of an AP, but it's important to know that the AP is whatever arbitrary point is chosen as a reference in any variant of the intercept method principle.

Working from a DR has at least two advantages. If your DR is reasonably accurate, say within a few miles, as it usually is in surface navigation, then you get intercept distances that are smaller by an order of magnitude compared the intercepts generated with an AP using special rules. Shorter intercepts allow us to plot at a larger scale where we can see how the LOPS intersect better, and it also avoids certain errors that result from the map projection when we are using long intercept distances. In addition, the "special rules" for an AP often require a separate AP position for each and every sight in a multi-body fix. This is both conceptually confusing for many navigators and harder to plot successfully. Generating intercepts from one single point, like the DR or any convenient "round number" position close to the DR, yields a cleaner plot and usually this plot can be drawn at a larger scale (more "zoomed in"). Both of these changes can impact the accuracy of the fix. The price, of course, is the interpolation, the "delta for both corrections" that you mentioned. And since that delta is only approximate in the tables and since it is an extra step prone to error, the supposed advantage is lost.

Notice that the advantages described here also apply to calculator realizations of the intercept method, but the disadvantages never arise. So if you need to use the intercept method, and if calculators are not counted among the Tools of Beelzebub, then working from the DR (or, as noted above, some convenient spot nearby) as the AP is the way to go. You get short intercepts and all plotting can done from one position.

Finally, when you have short intercepts using the intercept method, you can also afford reduced precision in the calculation of the azimuth of the body. With short intercepts, if the azimuth is off by a degree or several, it has limit impact on the fix. Of course, this is not an important advantage if you have a calculator at hand (though it can save keystrokes since you can round all inputs to, let's say, the nearest tenth of a degree), but if we're dealing with a partially or fully paper-based solution, then the azimuth can be looked up using simple graphical tools or very short tables. This can add a modest practical advantage to the process.

Frank Reed
Conanicut Island USA