NavList:

This is very basic to many on the list, but some of the newer or less experienced people on the list might find it useful.

When you are a vessel in motion while shooting a round of observations, the position of the vessel has changed between each sight. How much this motion affects your fix position depends greatly on the speed of the vessel,the difference between the course made good and the azimuth of the body; and finally, the time between your selected "fix time" and the observation.

As a body's azimuth approaches 90 degrees from the CMG (that is to say that the body is on your beam), the LOP's derived from this body will come closer and closer to parallel to the CMG. Advancing or retarding the lines will have a small effect on the actual fix. If you are in a hurry, you can usually safely skip adjusting LOP's shot on the beam.

On the other hand, bodies shot very close to fore and aft on a ship will render LOP's very close to perpendicular to the CMG and failing to advance these lines will adversely affect the fix in measurable amounts. I find it is always good to advance or retard these lines.

As for speed, the faster the vessel, the more change in position, so if you are on a 20 knot ship, your error from not advancing LOP's will be much greater than a vessel sailing at 3 knots.

At sea on commercial ships, "fix time" is usually selected to be on the top or bottom of the hours. There is no fixed rule for this, but it makes distance run calculations easier. If your sight is many minutes from the fix time, it is always a good idea to adjust the line. If your sight is within 6 minutes of the fix time, it can usually be safely left untouched unless the vessel is moving at a very high speed.

There are 2 methods of advancing/retarding LOP's. The first is to select the Assumed Position (AP) and take the azimuth and intercept then lightly plot the line. After this, take the formula Distance = Time x Speed. Where distance is nautical miles, times is in hours (minutes/60) and speed is in knots. You will find that time will be a fraction of an hour where 0.1 hours = 6 minutes. Once you have the distance to retard or advance to your pre-selected fix time, you shift the line along the course line towards the course if you are advancing the LOP and 180 degrees from the course if you are retarding the LOP the distance you calculated. The trouble with this method is that you have many LOP's near the fix position and it can get confusing.

My preferred method of advancing or retarding is to shift the AP of the body either in the direction of the CMG or 180 from the CMG depending on whether you are advancing or retarding, and then plot the azimuth and intercept to get a single LOP that has already been advanced. This makes for a nice, clean plot.

For an example.

A ship is heading south (180 deg) at 20 knots. A shot of polaris is taken at 1842 where the azimuth is 000 and the intercept is 3.0 Away. The assumed latitude is 30 deg North. If you have selected 1830 as the fix time, how do you retard this LOP?

Well first we will notice that the star is on our stern. This means that if we do not advance/retard the LOP, the error will be significant since the LOP will be a latitude line 90 degrees from our CMG of 180 degrees. So let us do the math.

D=S x T. The time difference between our fix time of 1830 and 1842 is 12 minutes. This is 12 minutes AFTER our fix time, so we need to retard the line from 1842 to 1830. -12/60 is -0.2 hours. Speed is 20 knots. This is a change of -4 nm. or 4 nm 180 degrees from the CMG so 180-180=000 degs. We need to move the original LOP 4 nm in a direction of 000 deg.

We plot the assumed latitude of 30 deg 00.0' North. Using the second method, we need to retard this sight 4 nm north. This would make our new assumed latitude 30 deg 04.0'N. Plot this new AP on the plotting sheet and then run the LOP as normal. Combine this line with others corrected to a common time will give us the most accurate fix we can get.

I hope this helps.

Jeremy
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