NavList:

----- Original Message -----

From: Peter Fogg

To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM

Sent: Friday, June 02, 2006 3:39 AM

Subject: When is it good?

Guy Schwartz wrote, on the 28 April:

“In the book 100 problems in Celestial Navigation. In the answer section problem 1-2 it says "The LOPs have more spread than we would like, but we rate the reliability as good"

In the perfect world all LOPs would cross at a given point, however the system is not perfect, therefore when they say it has more spread than they would like, how much spread is to much? Is there a certain distances that relate to excellent, very good, good, fair and unuseable?  Do these distances relate to the reliability of the sights.”

And I answered at the time. Now I would like to take advantage of being able to include simple examples to illustrate this, and add to my response at that time:

If these are two LOPs then the fix is at the intersection. Correctness is quite dependent, among other things, on azimuth accuracy. So its a good idea to shoot a third body if possible, ideally all three with a wide range of azimuths. The third LOP might look like this:

The navigator would probably feel quite encouraged by this small enclosing ‘cocked-hat’ and might take it as an indication of a fairly accurate round of sights, with the fix enclosed at the centre of the small triangle. And so it might be. On the other hand, that third LOP could be a blunder. If the mistake (poor sight, horizon, timing, calculation, take your pick) had not happened, or was rectified, that more correct LOP might look like this

 

Now the navigator might feel disappointed, as the encompassing LOPs have much more ‘spread’. But if the actual position is located within them then this is obviously a better, more accurate result despite the greater spread.

 

I guess the bottom line is that there is no hard and fast rule about LOP spread, although as a generality a smaller encompassed area is more encouraging than the converse.