NavList:

Joel:

I absolutely agree. 

You know your vessel's position when a bearing was taken.  When you advance the bearing, you estimate your vessel's position as accurately as you can (or, more accurately, the change in your vessel's position, since you are trying to establish/verify your vessel's position).   Perhaps it's only a DR position.  Even better, if you know of leeway or currents that might have affected your position, the use them to create an even more accurate EP. 

We have to crawl before we can walk.  I don't claim my "imagine that the sighted object clones" explanation is the most complete and comprehensive explanation of advancing a LOP.  Rather it's really intended as a first step, to give the student some intuitive basis for the whole idea of advancing a LOP.  Once the student grasps this, I think it's easy to then go on to more advanced issues such as course changes and current/leeway adjustments.   And I'll freely admit "clamp the bearing line to the vessel" analogy may be a more appropriate teaching vehicle for those situations.

Lu

LAS Office wrote:

John,

 

I do not believe in practice people using a running fix will ignore set and drift of current in plotting or the affect of leeway in a sailing vessel, or a vessel affected by a lot windage, if they have a reasonable way of estimating these forces. I suggest the example Lu posted was simplified for teaching purposes which gets students to understand the basis concept which is a good thing.

 

If the force of offset is unknown, one way to do this is the use of "back bearings" which are useful no matter what type of ship.

 

For the same reason, since you did not explain or show how you calculated the vector offset in determining your EP 1 which you carry forward to EP 2, you must have also simplified it for clarity of illustration.

 

Just a thought,

 

Joel Jacobs

 

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

From: John Karl

To: NavList@fer3.com

Sent: Sunday, December 13, 2009 11:25 AM

Subject: [NavList 11116] Why Not To Teach Running Fixes

Ah, the traditions of the sea.

For all the talk on how to teach running fixes, here's my 2 cents 
worth on why to not teach them at all -- they make no sense whatsoever.

The figure below shows that they operate under extremely ridiculous 
assumptions: They assume that the estimated DR track perpendicular to 
LOP1 is completely accurate and that the DR component parallel to LOP1 
is completely without value.  Can anyone on the List justify these two 
assumptions??

I recommend using the Estimated Position (EP) concept shown in the 
figure.  It fully honors the new LOP2 while retaining the information 
in the DR that is not contradicted by LOP2.

JK

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